Search

Laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down spectra of the A 1 ̃ 2 A ′ ′ / A 2 ̃ 2 A ′ − X ̃ 2 A ′ electronic transition of the calcium ethoxide (CaOC2H5) radical have been obtained under jet-cooled conditions. An essentially constant A ̃ 2 − A ̃ 1 energy separation for different vibronic levels is observed in the LIF spectrum, which is attributed to both the spin–orbit (SO) interaction and non-relativistic effects. Electronic transition energies, vibrational frequencies, and spin–vibrational eigenfunctions calculated using the coupled-cluster method, along with results from previous complete active space self-consistent field calculations, have been used to predict the vibronic energy level structure and simulate the recorded LIF/DF spectra. Although the vibrational frequencies and Franck–Condon (FC) factors calculated under the Born–Oppenheimer approximation and the harmonic oscillator approximation reproduce the dominant spectral features well, the inclusion of the pseudo-Jahn–Teller (pJT) and SO interactions, especially those between the A 1 ̃ 2 A ″ / A 2 ̃ 2 A ′ and the B ̃ 2 A ′ states, induces additional vibronic transitions and significantly improves the accuracy of the spectral simulations. Notably, the spin–vibronic interactions couple vibronic levels and alter transition intensities. The calculated FC matrix for the A 1 ̃ 2 A ′ ′ / A 2 ̃ 2 A ′ − X ̃ 2 A ′ transition contains a number of off-diagonal matrix elements that connect the vibrational ground levels to the levels of the ν8 (CO stretch), ν11 (OCC bending), ν12 (CaO stretch), ν13 (in-plane CaOC bending), and ν21 (out-of-plane CaOC bending) modes, which are used for vibrational assignments. Transitions to the ν21(a″) levels are allowed due to the pJT effect. Furthermore, when LIF transitions to the A ̃ -state levels of the CaOC-bending modes, ν13 and ν21, are pumped, A 1 ̃ 2 A ′ ′ / A 2 ̃ 2 A ′ → X ̃ 2 A ′ transitions to the combination levels of these two modes with the ν8, ν11, and ν12 modes are also observed in the DF spectra due to the Duschinsky mixing. Implications of the present spectroscopic investigation to laser cooling of asymmetric-top molecules are discussed. [ABSTRACT FROM AUTHOR]

Rotationally and fine-structure resolved B ̃ ← X ̃ laser-induced fluorescence (LIF) spectra of alkoxy radicals have been simulated with a "coupled two-states model" [J. Liu, J. Chem. Phys. 148, 124112 (2018)], in which the nearly degenerate X ̃ and à states are considered together. These two electronic states are separated by the "difference potential" and coupled by the spin–orbit (SO) interaction and the Coriolis interaction. Molecular constants determined in fitting the LIF spectra using the coupled two-states model provide quantitative insight into the SO and Coriolis interactions, as well as other intramolecular dynamics, including the pseudo-Jahn–Teller effect. The spectroscopic model also allows semi-quantitative prediction of effective spin-rotation constants using molecular geometry and SO constants, which can be calculated ab initio with considerable accuracy. The dependence of fit values of molecular constants on the size and conformation of alkoxy radicals is discussed. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the à 2 E − X ̃ 2 A 1 electronic transition of the calcium methoxide (CaOCH3) radical have been obtained under jet-cooled conditions. Complete active space self-consistent field and coupled-cluster calculations on the free radical were performed to aid the assignment of vibronic transitions observed in the LIF/DF spectra. In addition to dominant spectral features that are well reproduced by vibrational frequencies and Franck-Condon (FC) factors calculated ab initio, the FC matrix for the à 2 E − X ̃ 2 A 1 electronic transition contains considerable off-diagonal elements that connect (i) the CaO-stretch (ν4) mode and non-CaO stretch modes and (ii) the asymmetric CaOC stretch (ν3) and the CaOC bending (ν8) modes. The Jahn-Teller and pseudo-Jahn-Teller interactions involving the Ã2E state as well as the spin-orbit interaction induce additional vibronic transitions that are not allowed under the Born-Oppenheimer approximation. Additionally, anharmonic vibrational terms in the ground state induce transitions that are forbidden in the harmonic-oscillator approximation. Spin-orbit splitting has been observed for several vibrational levels of the Ã2E state, and an essentially constant value was measured at all levels accessed in the LIF experiment. Implications of the present spectroscopic investigation to the proposed schemes of laser-cooling MOCH3 (M = alkaline earth metals) molecules and detection of time-reversal-symmetry-violating interactions are discussed. [ABSTRACT FROM AUTHOR]

The òE" ← X2A'2 spectrum of NO3 radical from 7550 cm-1 to 9750 cm-1 has been recorded and analyzed. Our spectrum differs from previously recorded spectra of this transition due to jet-cooling, which narrows the rotational contours and eliminates spectral interference from hot bands. Assignments of numerous vibronic features can be made based on both band contour and position including the previously unassigned 3¹0 band and several associated combination bands. We have analyzed our spectrum first with an independent anharmonic oscillator model and then by a quadratic Jahn-Teller vibronic coupling model. The fit achieved with the quadratic Jahn- Teller model is excellent, but the potential energy surface obtained with the fitted parameters is in only qualitative agreement with one obtained from ab initio calculations. [ABSTRACT FROM AUTHOR]

The jet-cooled Ã-X near IR origin band spectra of the G1G2G3 conformer of four β-hydroxyethylperoxy isotopologues, β-HEP (HOCH2CH2OO), β-DHEP (DOCH2CH2OO), β-HEP-d4 (HOCD2CD2OO), and β-DHEP-d4 (DOCD2CD2OO), have been recorded by a cavity ringdown spectrometer with a laser source linewidth of ∼70 MHz. The spectra of all four isotopologues have been analyzed and successfully simulated with an evolutionary algorithm, confirming the cyclic structure of the molecule responsible for the observed origin band. The analysis also provides experimental à and X state rotational constants and the orientation of the transition dipole moment in the inertial axis system; these quantities are compared to results from electronic structure calculations. The observed, broad linewidth (Δν > 2 GHz) is attributed to a shortened lifetime of the à state associated with dynamics along the reaction path for hydrogen transfer from the OH to OO group. [ABSTRACT FROM AUTHOR]

Rotationally resolved laser induced fluorescence and stimulated emission pumping Ã2A1-X2E spectra, along with pure rotational spectra in the 153-263 GHz region within the E3/2 component of the ground state in asymmetrically deuterated methoxy radicals CH2DO and CHD2O have been observed. The combined data set allows for the direct measurement with high precision of the energy separation between the E1/2 and E3/2 components of the ground state and the energy separation between the parity stacks in the E3/2 component of the ground state. The experimentally observed frequencies in both isotopologues are fit to an effective rotational Hamiltonian accounting for rotational and spin-rotational effects arising in a near-prolate asymmetric top molecule with dynamic Jahn-Teller distortion. Isotopic dependencies for the molecular parameters have been successfully implemented to aid the analysis of these very complex spectra. The analysis of the first and second order contributions to the effective values of molecular parameters has been extended to elucidate the physical significance of resulting molecular parameters. Comparisons of measured parameters, e.g., spin-orbit coupling, rotational and spin-rotation constants, are made among the 5 methoxy isotopologues for which data is now available. Comparisons of experimental results, including the derived geometric structure at both the C3v conical intersection and at the Jahn-Teller distorted minima, are made with quantum chemistry calculations. [ABSTRACT FROM AUTHOR]

The Ã-X electronic absorption spectrum of vinoxy radical has been investigated using room temperature cavity ringdown spectroscopy. Analysis of the observed bands on the basis of computed vibrational frequencies and rotational envelopes reveals that two distinct types of features are present with comparable intensities. The first type corresponds to “normal” allowed electronic transitions to the origin and symmetric vibrations in the à state. The second type is interpreted in terms of excitations to asymmetric à state vibrations, which are only vibronically allowed by Herzberg–Teller coupling to the B state. Results of electronic structure calculations indicate that the magnitude of the Herzberg–Teller coupling is appropriate to produce vibronically induced transitions with intensities comparable to those of the normal bands. [ABSTRACT FROM AUTHOR]

We have recorded high resolution, partially rotationally resolved, jet-cooled cavity ringdown spectra of the origin band of the Ã-X electronic transition of both the G and T conformers of the perproteo and perdeutero isotopologues of the ethyl peroxy radical, C2H5O2. This transition, located in the near infrared, was studied using a narrow band laser source (<=250 MHz) and a supersonic slit-jet expansion coupled with an electric discharge allowing us to obtain rotational temperatures of about 15 K. All four spectra have been successfully simulated using an evolutionary algorithm approach with a Hamiltonian including rotational and spin-rotational terms. Excellent agreement with the experimental spectra was obtained by fitting seven molecular parameters in each ground and the first excited electronic states as well as the band origin of the electronic transition. This analysis unambiguously confirms the assignment of the lower frequency origin band to the G conformer and the higher frequency one to the T conformer. [ABSTRACT FROM AUTHOR]

New experimental data have been obtained for the methoxy radical by observing at high-resolution laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) transitions between the X 2E and lowest excited à 2A1 state. The SEP transitions were from the à state (pumped from the X 2E3/2 spin-orbit component) to the X 2E1/2 spin-orbit component. These data for the first time directly connect with high precision the spin-orbit components of the X 2E ground state of CH3O. Surprisingly these new SEP observations are inconsistent with predictions of the X state structure based on long-standing analyses primarily based on the microwave spectra of ground state CH3O. It is found that all the experimental data can be understood consistently when the previously accepted value of the spin-orbit coupling constant is adjusted and the reflection parity assignments in the X state are reversed. The latter action changes the sign of a number of reflection-parity-dependent parameters in the X state. The ramifications of the changes and the physical interpretation of the resulting parameters are discussed in some detail. [ABSTRACT FROM AUTHOR]

Theoretical calculations are performed for the X 2E2″ and à 2E3″ states of the cycloheptatrienyl (tropyl) radical C7H7. An important goal of these calculations is to predict and to guide the analysis of the experimentally observed à 2E3″-X 2E2″ electronic spectrum. Vibrational frequencies of the tropyl radical at the conical intersection and stationary points of its X and à state Jahn-Teller distorted potential energy surfaces are given. Spectroscopically obtainable parameters describing the Jahn-Teller effect are calculated for the X and à electronic states. Additionally, the stabilization energies for the X and à states are computed at the CASSCF(7,7) and EOMEA-CCSD levels of theory using various basis sets. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence (LIF) and laser-excited dispersed fluorescence (LEDF) spectra of the cycloheptatrienyl (tropyl) radical C7H7 have been observed under supersonic jet-cooling conditions. Assignment of the LIF excitation spectrum yields detailed information about the Ã-state vibronic structure. The LEDF emission was collected by pumping different vibronic bands of the à 2E3″←X 2E2″ electronic spectrum. Analysis of the LEDF spectra yields valuable information about the vibronic levels of the X 2E2″ state. The X- and Ã-state vibronic structures characterize the Jahn-Teller distortion of the respective potential energy surfaces. A thorough analysis reveals observable Jahn-Teller activity in three of the four e3′ modes for the X 2E2″ state and two of the three e1′ modes for the à 2E3″ state and provides values for their deperturbed vibrational frequencies as well as linear Jahn-Teller coupling constants. The molecular parameters characterizing the Jahn-Teller interaction in the X and à states of C7H7 are compared to theoretical results and to those previously obtained for C5H5 and C6H6+. [ABSTRACT FROM AUTHOR]

The nearly rotationally resolved spectrum of the à 2A′←X 2A″ 000 transition of perdeutero methyl peroxy near 1.35 μm has been studied via pulsed cavity ringdown spectroscopy. Albeit, this is a weak transition, it is possible to observe the spectrum under jet-cooled conditions (≈15 K) by combining a source of narrow-bandwidth radiation (≈250 MHz) with a supersonic slit-jet expansion incorporating an electric discharge. The near infrared radiation was obtained by using stimulated Raman scattering and a pulsed, nearly Fourier-transform-limited Ti:sapphire amplifier seeded by a scanable cw Ti:sapphire ring laser. The experimental spectrum has been fitted using a model Hamiltonian that includes the rigid body rotation of an asymmetric top and the spin-rotation interaction. An excellent quality fit was obtained resulting in the determination of 15 molecular parameters characterizing the à and X states. Other results reported for CD3O2 include an estimate of the radical concentration and the vibronic transition dipole from the observed absorption intensities. Details about the spectral linewidths are also discussed. [ABSTRACT FROM AUTHOR]

Recent experiments have prompted a theoretical investigation of the effect of methyl rotation on the Ã-X electronic spectrum of the CH3O2 and CD3O2 radicals. Quantum chemistry calculations have mapped the potential for the methyl rotation. Using these results, we calculate the torsional eigenvalues for both the à and X states and simulate the Ã-X spectrum. We find that the simulation captures the salient features of the spectrum. These features include torsional sequence structure, whose band contours change dramatically as the lower level nears the barrier, as well as atypical torsional transitions occurring from levels near the top and above the barrier. “Experimental” barrier heights are deduced for both the X and à states of methyl peroxy by modestly scaling the calculated potential to best reproduce the observed spectra. [ABSTRACT FROM AUTHOR]

We have recorded several rovibronic bands of CH3OO and CD3OO in their Ã←X transitions in the range of 1.18–1.40 μm with the cavity ringdown technique. While the electronic origins for these species have been reported previously, many newly observed rovibronic bands are described here. The experimental vibrational frequencies (given as ν in the unit cm-1 in this paper) for the COO bending (ν8) and COO symmetric stretching (ν7) modes in the à state are 378 and 887 cm-1 for CH3OO, and 348 and 824 cm-1 for CD3OO, respectively. In addition, two other vibrational frequencies were observed for the à state of CD3OO, namely, ν5 (954 cm-1) and ν6 (971 cm-1). These experimental vibrational frequencies for the à state of both CH3OO and CD3OO are in good agreement with predictions from quantum-chemical calculations at the UB3LYP/aug-cc-pVTZ level. The enhanced activity of the ν5 vibrational mode in CD3OO is rationalized by mode mixing with the ν7 mode, as supported by calculations of multidimensional Franck-Condon factors. In addition, many hot bands involving the methyl torsional mode (ν12) are observed for both normal and deuterated methyl peroxy. These bands include the “typical” sequence transitions and some “atypical” ones due to the nature of the eigenvalues and eigenfunctions which are a consequence of the low, but very different, torsional barriers in the X and à states. In addition, the 1222 band in CH3OO and the 1233 band in CD3OO show quite different structures than the origin bands, an effect which results from tunneling splittings comparable to the rotational contour. [ABSTRACT FROM AUTHOR]

Extensive ab initio calculations were performed for the X 2E′ and à 2E″ states of Ag3, using a newly constructed basis set for Ag. An important goal of these calculations is to guide the analysis of the experimentally observed à 2E″-X 2E′ electronic spectrum. Vibrational frequencies of Ag3 for both the X and à states are reported. Spectroscopically obtainable parameters describing the Jahn-Teller effect are calculated for the X and à states. The magnitude of the spin-orbit effects for this relativistic system was also calculated for the X 2E′ and à 2E″ states. Using all this information, the X-à electronic spectrum is predicted for Ag3. Additionally, the geometries and symmetries of the global minima and saddle points as well as the barrier to pseudorotation around the moat of the potential energy surface are determined for both states. [ABSTRACT FROM AUTHOR]

The laser-excited, jet-cooled à 2E″-X 2E′ electronic spectrum of the silver trimer yields detailed information about its Ã- and X-state vibronic structure. Following extensive parameter fitting, the absorption and emission spectra are simulated and the bands are assigned. The Jahn-Teller analysis includes both linear and quadratic coupling terms, considered simultaneously with spin-orbit coupling. The spin-orbit splitting is shown to be largely quenched in both the à and X electronic states. The Jahn-Teller analysis of the à and X vibronic structures reveals the distortion of their corresponding potential energy surfaces. [ABSTRACT FROM AUTHOR]

We have recorded five high resolution (200 MHz), rotationally resolved, vibrational bands of the ...-... electronic transition of 2-butoxy. Two bands of the 2-butoxy spectrum have been rotationally analyzed and assigned to two different geometrical conformers of the molecule. The analyses allow the determination of the six experimental rotational constants defined by the geometry of the species in the ground (...) and excited (...) electronic states and also four spin-rotation constants for the X electronic state of the conformers. Comparison of the experimental rotational constants with the results of ab initio computations provides unambiguous conformational assignment of these bands. This approach can be extended to assign two other spectral bands to the third 2-butoxy conformer. [ABSTRACT FROM AUTHOR]

The change of the 7-azaindole-water cluster structure upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of five different vibronic bands. A total of 105 emission band intensities were fitted, together with the changes of rotational constants of one isotopomer. These rotational constants have been obtained from a fit to the rovibronic contour of the cluster. The geometry change upon electronic excitation to the ππ* state can be described by a strong and asymmetric shortening of the hydrogen bonds and a deformation of both the pyridine and the pyrrole rings of 7-azaindole. The resulting geometry changes are interpreted on the basis of ab initio calculations. [ABSTRACT FROM AUTHOR]

Dispersed fluorescence (DF) spectra of 1-propoxy, 1-butoxy, 2-propoxy, and 2-butoxy radicals have been observed under supersonic jet cooling conditions by pumping different vibronic bands of the B-X laser induced fluorescence excitation spectrum. The DF spectra were recorded for both conformers of 1-propoxy, three conformers of the possible five of 1-butoxy, the one possible conformer of 2-propoxy, and two conformers of the possible three of 2-butoxy. Analysis of the spectra yields the energy separations of the vibrationless levels of the ground X and low-lying à electronic state as well as their vibrational frequencies. In all cases, the vibrational structure of the DF spectra is dominated by a CO stretch progression yielding the νCO stretching frequency for the X state and in most cases for the à state. In addition to the experimental work, quantum chemical calculations were carried out to aid the assignment of the vibrational levels of the X state and for some conformers the à state as well. Geometry optimizations of the different conformers of the isomers were performed and their energy differences in the ground states were determined. The results of the calculation of the energy separations of the close-lying X and à states of the different conformations are provided for comparison with the experimental observations.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The laser-induced fluorescence and laser-excited dispersed fluorescence spectra of the cyclohexoxy radical has been observed under two sets of free-jet-cooling conditions, characterized by rotational temperatures of approximately 1 and 100 K. Although five conformers of cyclohexoxy are possible, it appears that all presently observed spectral bands can be accounted for by a single one. All cold spectral bands are assigned to the &Btilde; ­ &Xtilde; electronic transition of the cyclohexoxy radical. Transitions to both ′ and ″ &Btilde; state vibrational levels are observed and allowed due to a substantial pseudo-Jahn­Teller effect in the &Xtilde; state. Hot bands are also observed, which we attributed to transitions to the &Btilde; state from the low-lying à electronic state. Analysis of the spectra yields vibrational frequencies for the &Xtilde;, Ã, and &Btilde; states as well as the energy separations of their vibrationless levels. [ABSTRACT FROM AUTHOR]

Five bands of the &Btilde;-&Xtilde; laser induced fluorescence spectrum of jet-cooled 1-propoxy radical have been recorded with a spectral resolution of ≈200 MHz. The resolved rotational and fine structure of these bands has been assigned and analyzed providing rotational constants for both the &Xtilde; and &Btilde; states as well as components of the electron spin-rotation tensor in the &Xtilde; state. By comparison of these constants with ones obtained from quantum chemistry calculations, two bands have been assigned to the gauche (G) conformer of 1-propoxy and 3 bands to the trans (T) conformer. The spectrum of each conformer abruptly terminates after the excitation of a single C-O stretch. [ABSTRACT FROM AUTHOR]

The rotational structure of the pseudorotational (PR) band n = 0←n=2 has been observed in jet-cooled tetrahydrofuran in the 170-360 GHz frequency range. The observed transitions were analyzed together with the previously obtained microwave data of Meyer and co-workers [R. Meyer, J. C. Lopes, J. L. Alonso, S. Melandri, P. G. Favero, and W. Caminati, J. Chem. Phys. 111, 7871 (1999)]. The experimentally observed transitions provide direct spectroscopic evidence of the symmetry ordering of the lowest four observed PR states. Based on the symmetry properties of the pseudorotational states involved in this study, an analytical model of the potential energy surface (PES) along the pseudorotational path has been proposed that provides a consistent explanation of all the observed transition frequencies, including those from the early IR work. In addition, an analysis of the variation of the rotational constants of the molecule in different PR states has been performed using the proposed model. This analytical PES and the derived rotational constants are compared to the results of it ab initio calculations. A discussion of the results obtained by different methods is given. [ABSTRACT FROM AUTHOR]

Ab initio calculations have been performed for the cations of benzene, C[sub 6]H[sub 6], and its fluorinated analogs, C[sub 6]F[sub 6] and C[sub 6]H[sub 3]F[sub 3]. Calculated molecular parameters characterizing the Jahn-Teller potential energy surface (PES) are very consistent with those derived from the spectra of C[sub 6]F[sup +, sub 6] and C[sub 6]H[sub 3]F[sup +, sub 3]. However the calculated Jahn-Teller stabilization energy for the benzene cation is roughly three times greater than that previously reported experimentally. With the aid of the calculated values, a more complete analysis of the available spectral data for C[sub 6]H[sup +, sub 6] and C[sub 6]D[sup +, sub 6] is performed, with an emphasis on the data from ZEKE experiments and IR spectra of the Ar·C[sub 6]H[sup +, sub 6], Ne · C[sub 6]H[sup +, sub 6], and Ar·C[sub 6]D[sup + sub 6] complexes. The comprehensive analysis reveals Jahn-Teller activity in 3 e[sub 2g] modes for C[sub 6](H/D)[sup +, sub 6] and provides values for their vibrational frequencies, linear and quadratic Jahn-Teller coupling constants, as well as quadratic coupling constants for several other degenerate modes. These new molecular parameters are generally in good agreement with the corresponding values derived from the ab initio calculations and produce a total Jahn-Teller stabilization energy in good agreement with the computed value. [ABSTRACT FROM AUTHOR]

Ab initio calculations are performed for the X˜ [sup 2]E[sub 1][sup ″] and A˜ [sup 2]A[sub 2][sup ″] states of the cyclopentadienyl radical. An important goal of these calculations is to guide the analysis of the experimentally observed A˜ [sup 2]A[sub 2][sup ″]- X˜ [sup 2]E[sub 1][sup ″] electronic spectrum. Vibrational frequencies for both the X˜ and A˜ state are reported. Large changes in frequency between the states for out-of-plane vibrations are found, leading to the expectation that overtones of these modes will appear strongly in the spectrum. Additionally, spectroscopically obtainable parameters describing the Jahn-Teller effect are calculated for the X˜ state. Using all this information the X˜-A˜ electronic spectrum is predicted for both C[sub 5]H[sub 5] and C[sub 5]D[sub 5]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The laser excited, jet-cooled A˜ [sup 2]A[sub 2][sup ″]-X˜ [sup 2]E[sub 1][sup ″] electronic spectrum of the cyclopentadienyl radical yields detailed information about the vibronic structure of both its A˜ and X˜ states. A straightforward assignment of the A˜ vibronic structure is presented. The X˜ state vibronic structure reveals a comprehensive picture of the Jahn-Teller distortion of its potential energy surface. The molecular parameters characterizing the Jahn-Teller interaction provide the stabilization energy and distorted geometry, which are compared to previous experimental and ab initio results. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the high-resolution laser-induced fluorescence (LIF) spectra of the Kr...OH van der Waals complex and its deuterated analog. Development of an empirical potential energy surface for the excited state using the rotational constant information from the high-resolution data.

Reports on the development of an empirical potential energy surface for the complexes from the experimentally observed vibrational and rotationary energy levels with the results of ab initio calculations being used to provide initial estimates of the values of the parameters in teh empirical potential. Vibrational energies and rotational constants resulting from the fit surface.

Investigates dark vibronic levels in the A[sup 2]E electronic state of the CdCH[sub 3] radical by using the fluorescence depletion spectroscopy technique. Laser-induced fluorescence spectrum of the radical; Spin-orbit splitting of the origin; Symmetric modes; Assignment of the degenerate mode v[sub 6] and the remainder of the spectrum; Lifetime measurements.

Doppler shifts of emissions from neutral species have been observed for the first time in a positive column of a dc glow discharge. For a detailed study, we have chosen emissions of H2 and its isotopomers in a hydrogen discharge using a high resolution Fourier-transform spectrometer. Experimetal evidence supports the conclusion that the observed shifts derive from momentum transfer during electron-molecule collisions that produce the emissive excited states. A theoretical model has been formulated to relate Doppler shifts to the drift velocity of electrons in the discharge as well as to the momentum transfer of electron-molecule collisions. Predictions based on the model are consistent with the experimental observations. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Doppler shifts of the emissions from the A–X and B–X transitions of N+2 ions in a He glow discharge are measured using the method of Fourier transform emission (FTE) spectroscopy. The magnitudes and pressure dependencies of the Doppler shifts of rovibronic lines of the two electronic transitions are distinctly different mainly due to the difference in the lifetimes of the excited states. By fitting the data to a theoretical model, the cross sections for He–N+2 collisions (both for velocity-changing and quenching collisions) are obtained. The data from the short-lived B 2Σ+u state of N+2 in the low pressure (no-collision) regime lead to the determination of the discharge electric field. The Doppler modulated spectra demonstrate that the FTE method is a useful spectroscopic tool for discriminating between ionic and neutral transitions. [ABSTRACT FROM AUTHOR]

Simultaneous laser-induced fluorescence (LIF) spectra and time-of-flight mass spectra have been recorded for ionic clusters, C6F+6·Rn where R=He, Ne, and Ar. These spectra span the regime of clusters extending from the isolated ion to the ion located in the corresponding inert-gas matrix (except He). The conclusions of these studies include the following. Abundant clusters with n=1 and 2 exist in symmetrical forms with one atom above and below the benzene plane. Such configurations appear, however, to be evolutionary dead ends with respect to the ultimate matrix structure. Rather, the latter likely corresponds to several inert-gas atoms sharing more or less equally the cationic charge on each side of the ring. From our results, it may be speculated that most of the essential features of the matrix LIF spectra are obtained with the completion of what is roughly the first solvent shell in the cluster, 6–10 atoms, depending upon the inert gas. [ABSTRACT FROM AUTHOR]

The B 2A2u←X 2E1g(0) laser induced fluorescence excitation spectrum of the hexafluorobenzene cation in a free jet expansion has been measured from 21 500 to 22 580 cm-1. The spectrum displays a number of strong transitions involving the totally symmetric and the Jahn–Teller active vibrations. In addition, many weak transitions involving other vibrational modes are observed using high laser powers. These weak, interspersed transitions are readily detected because the ion is cooled in a supersonic expansion which results in sharp, well resolved lines. These weak transitions derive their intensities by a variety of mechanisms, the most novel of which depends on the presence of mixed cross-quadratic nuclear potential terms in the ground (2E1g) state. It is shown that these terms permit the observation of transitions from the vibrationless level of the ground state to combination levels of the excited state involving two modes of different symmetries. The identification of transitions of this type provides the first experimental demonstration of the occurrence of mixed cross-quadratic nuclear potential terms in spatially degenerate electronic states. The analysis of the spectrum has allowed overall the determination of fundamental frequencies for 16 of the 20 normal modes of C6F+6 in its B 2A2u electronic state. [ABSTRACT FROM AUTHOR]

Examines the high resolution laser induced fluorescence spectra of the hydroxyl radical inert gas complexes. Inclusion of rotation, fine and hyperfine structure, spin-rotation interactions and parity splitting in the analysis of bands; Determining a number of molecular parameters.

Various types of strongly enhanced infrared vibrational transitions, with transition moments comparable to those characteristic of allowed electronic transitions in visible or UV spectra, may ensue in spatially degenerate electronic states of nonlinear molecules. The intimate correlation between the intensity of the enhanced infrared vibrational transitions and the anomalous pure rotational microwave spectra and the Stark effect is elucidated. The rules of Child and Longuet-Higgins with regard to infrared activity and infrared intensity enhancements in electronically degenerate states require revision and extension when properly considering the ramifications of the Jahn–Teller effect and invoking the action of mixed quadratic nuclear potential terms. Significantly, the enhancements of IR transition moments should allow the observation of symmetrical free radicals and molecular ions whose laboratory concentrations are often below the level of detectability for ‘‘normal’’ IR transition strengths. In addition, these effects could lead to multi-enhancement effects in nonlinear infrared and Raman vibrational spectra as well as enhanced radiative and nonradiative vibrational relaxations. [ABSTRACT FROM AUTHOR]

The MgCH3 radical was produced by a laser ablation/photolysis technique in a cold supersonic free-jet expansion and probed by laser induced fluorescence. Rotationally resolved spectra for both spin-orbit components of the A 2E←X 2A1 electronic transition have been recorded. The analysis of these spectra yields the rotational constants of MgCH3 and therefrom a structure for the radical is proposed. A comparison is made among a series of alkyl organometallic radicals. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ba+–Ar open-shell ionic complexes were produced in a pulsed free-jet expansion. The dispersed emission and both the low and high resolution A 2Π–X 2Σ+ excitation spectra of the Ba+–Ar complex are reported. The data obtained were used to construct potentials for the ground and excited states. A simple quantum mechanical model was introduced in order to simulate the experimentally measured potentials. The model potential is used to estimate the dissociation energy of the ground 2Σ+ state. This value, when combined with the spectral red shift, allows the dissociation energies of the two components of the excited 2Π state to be determined. The same electrostatic interaction model also explains the observed angular momentum coupling scheme as well as the much stronger binding in the excited 2Π state. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The one-color (1+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of the S1(1B2) state of chlorobenzene has been recorded and is similar to that obtained by other workers. Zero-kinetic-energy (ZEKE) photoelectron spectroscopy was then used to probe the vibrational levels in the ground electronic state of the cation (I+0) using a two-color photoionization scheme via the S1 electronic state. By using different intermediate vibrational levels in the S1 state, different vibrations could be accessed in the ion. Vibrational symmetry selection rules for the I+0←S1 ionization appear to hold well. Exciting through different S1 vibrational levels has revealed the probable mixing of the S1 normal coordinates in I+0. A previously-identified Fermi resonance in the S1 state is also confirmed by the ZEKE spectra. The adiabatic ionization energy is measured as 73 170±5 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Rotationally resolved spectra have been recorded and analyzed for both spin–orbit components of the 000 and 410 (ν4= Ca–C5H5 stretch) vibrational bands of the A 2E1←X 2A1 transition of the organometallic free radical CaC5H5. The radicals were prepared with a laser ablation/photolysis technique, then probed in a supersonic free-jet expansion. Intensity variations from transitions originating from various K‘ levels confirm the C5v nature of the system with the Ca atom centered over the cyclopentadienyl ring. A complete structure could not be uniquely deduced from the rotational constants determined in the global fit of the two vibrational bands but reasonable assumptions for the C5H5 ring enable the Ca–C5H5 ring distance to be determined to be 2.333(+13-23) Å in the ground state which shortens by 0.060(1) Å in the excited state. Other fitted molecular constants indicate that Jahn–Teller distortion effects are unmeasurably small and that the electronic orbital angular momentum in the excited state is essentially unquenched with ζt=0.9678(3). © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Magnetic hyperfine interactions in the ground and first excited states of CdH and CdCH3 were studied using high resolution electronic spectroscopy. Hyperfine splittings associated with the H nucleus were observed in the X state of CdH; hyperfine splittings associated with 111Cd and 113Cd were observed in both the ground and first excited states of CdH and CdCH3. The hyperfine parameters of CdH were found to be similar to the corresponding parameters of CdCH3. Comparison of the ground state molecular hyperfine constants with values determined in an Ar matrix electron paramagnetic resonance (EPR) study [L. B. Knight, Jr. and W. Weltner, Jr., J. Chem. Phys. 55, 2061 (1971)] indicates clearly a shift of unpaired electron spin density towards Cd, in the matrix. The excited state molecular hyperfine constants indicate that the unpaired electron essentially resides in a Cd 5pπ orbital. [ABSTRACT FROM AUTHOR]

ZnCH3 and CdCH3 radicals have been prepared in a cold supersonic free jet expansion and their laser-induced-fluorescence spectrum recorded for the A 2E←X 2A1 electronic transition. These spectra show well resolved rotational and spin structure, which has been completely analyzed. This analysis yields the rotational constants and the components of the spin–rotation tensors in the A and X states of both radicals. The observed constants are discussed in terms of the electronic structure of the radicals. It is demonstrated that the upper F2 spin–orbit component of the A 2E state of CdCH3 is strongly perturbed by another, dissociative electronic state. This leads to some predissociation of the A 2E3/2 component and a broadening of its lines. The rotational and fine structure in this state is also quite perturbed leading to an unusual, but still interpretable spectrum. [ABSTRACT FROM AUTHOR]

The laser excitation spectrum A (0,00,vS’)-X (0,00,0), has been observed for the Ar·SH complex. Rotational constants and bond lengths have been obtained for vibrational levels of the A and X states. A remarkable lengthening of the A state natural lifetime is observed upon Ar complexation. [ABSTRACT FROM AUTHOR]

The rotationally resolved, laser induced fluorescence, excitation spectra of the partially deuterated cyclopentadienyl radicals, C5H4D and C5HD4, have been observed at low temperature in a supersonic free jet expansion. The observed electronic transition in the uv region corresponds to the A 2A‘2[larger_closed_square]X 2E‘1 transition in the symmetric cyclopentadienyl isotopomers with D5h symmetry. In the reduced C2v symmetry of the C5HD4 and C5H4D isotopomers, this electronic transition splits into two distinct vibronic bands, separated by about 9 cm-1, which arise from the two vibronic components X1 and X2 into which the X state is resolved when the symmetry is lowered. In C5H4D the ground X1 state has 2A2 symmetry and a permanently distorted, elongated allyl-like structure while the low-lying X2 state has 2B2 symmetry and a compressed dienelike structure. The symmetries of the energy levels and the distortions are reversed for the C5HD4 species. A detailed theoretical model is developed to describe the splitting and the rotational structure of the X1 and X2 states. Application of this model yields a precise value for the alternation of the C–C bond lengths in the distorted structures. This value is shown to be the same as that describing the dynamically pseudorotating, Jahn–Teller distorted structures in the D5h isotopomers. Additionally, the theory explains anomalous values of the observed inertial defects and relates them to physically meaningful quantities for all the isotopomers in both their X1 and X2 states. Our results are compared to previous experimental work and ab initio calculations on the cyclopentadienyl radical. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence spectra with resolved rotational, fine, and hyperfine structure have been observed for the U bands of the A←X electronic transition of the Ar·OH/D complex. The appearance of these spectra are very different from the previously analyzed A band system of the complex. Detailed analysis of the spectra reveals that their spectral differences are caused by changes in values of molecular parameters (due to the different regions of the potential surface sampled in the A state) and their resulting effect upon the coupling of the angular momenta in the complex. Numerical results are given for the molecular parameters in the A state levels examined in these experiments and the implication of these values for the geometry and the potential surface of the complex are discussed. [ABSTRACT FROM AUTHOR]

Several inert-gas complexes of the free radical cyclopentadienyl and its methyl derivative (C5H5·He, C5H5·Ne, C5H5·He2, and CH3–C5H4·He2, and CH3–C5H4·He) have been studied at low temperature via their laser induced fluorescence spectra in a supersonic free jet expansion. The rotational structures of the near-uv electronic transitions of the radicals have been resolved and analyzed. These analyses lead to precisely defined geometries for the radicals. In contrast to inert-gas, closed-shell aromatic complexes, the inert-gas radical ring bond lengths are found to be strongly dependent upon electronic state and methyl substitution. Additional molecular parameters were measured in the complexes including the electronic angular momentum, the (2,2) interaction parameter for the C5H5 complexes, and for the methyl derivative, the barrier to methyl rotation. These are compared to their values in the uncomplexed radicals. [ABSTRACT FROM AUTHOR]

The high resolution electronic spectrum of Ne·OH has been recorded in a supersonic free jet expansion using the laser-induced fluorescence technique. From an analysis of the spectrum which yields rotational constants, we are able to obtain Ne·OH bond lengths in several vibrational (hindered rotor) levels of the excited state and the vibrationless level of the ground state. We also measure the Fermi contact constant in the A state which is, unlike Ar·OH, insignificantly perturbed from the value in the OH monomer. However, we now measure a parity doubling of the X state rotational levels which is tenfold larger than the upper limit we established for such an interaction in Ar·OH. We interpret these latter measurements to imply weaker and more isotropic bonding in Ne·OH compared to Ar·OH in both electronic states. [ABSTRACT FROM AUTHOR]

A number of vibrational bands of the A 2Σ+X 2Π electronic spectrum of both ArOH and ArOD have been investigated by laser induced fluorescence with a high-resolution, pulsed laser system yielding linewidths ≲250 MHz in the UV. This spectrum not only displays completely resolved rotational structure, but also fine and hyperfine structure. The hyperfine constants and precise interatomic distances derived from the rotational constants provide a very interesting picture of the electronic and geometric structure of the complex. The bonding is incipiently chemical in the A state with clear evidence for at least some electronic reorganization between Ar and the open-shell OH radical in the complex. Conversely, the X state appears to be bound almost solely by physical van der Waals interactions characteristic of systems containing only closed-shell species. [ABSTRACT FROM AUTHOR]

The rotationally resolved, electronic, excitation spectra of supersonically cooled methylcyclopentadienyl radicals, CH3–C5H4 and CD3–C5H4, have been obtained. Analysis of these spectra characterizes both the ground and electronically excited states involved in this transition. The ground state barrier to internal rotation of the methyl group is very high compared to similar closed-shell species. Additionally it shows a strong isotopic dependence and increases in magnitude further in the excited state. The doubly degenerate 2E‘1 ground state of C5H5 is split into two components, X 2B2 and A 2A2. The rotational analysis shows that the cyclopentadienyl ring undergoes a significant, static distortion from a regular pentagon. The measured distortion is consistent with and related to the dynamic Jahn–Teller distortion in C5H5 itself. [ABSTRACT FROM AUTHOR]

The triatomic oxide Al2O was prepared by oxidation of laser vaporized aluminum, or by direct vaporization of aluminum oxide, and detected by laser induced fluorescence. The fluorescence excitation and resolved emission spectra of a transition located near 38 249 cm-1 are consistent with a linear, centrosymmetric Al–O–Al structure. Analysis of the spectrum yields an AlO bond length of 0.164 nm and values of 525 (Σ+g), 99 (Πu), and 992 cm-1 (Σ+g) for the ground state fundamental vibrations, in good agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

A laser vaporization/photolysis technique for obtaining spectra of jet-cooled organometallic molecules is reported. It makes use of an excimer laser which vaporizes a metal sample and simultaneously photolyzes an organic precursor passing over the surface of the metal. The metal atoms and organic fragments then react to yield the organometallic species. In principle, this approach could be used to prepare a wide variety of gas-phase compounds of any metal for spectroscopic study. To illustrate its capabilities, laser excitation spectra of the monocyclopentadienide and monomethyl derivatives of Ca and Cd are presented. In the case of cadmium cyclopentadienide, this report constitutes the first spectroscopic observation of this molecule. [ABSTRACT FROM AUTHOR]

The nearly completely rotationally resolved electronic spectra of two aromatic organic ions have been obtained. Rotationally cold ions, C6F+6 and C6F3H+3, are produced by laser ionization in a supersonic free jet expansion and probed via laser induced fluorescence with a very high resolution pulse-amplified cw ring dye laser. The spectra are analyzed to obtain band origins, rotational constants, Coriolis and Jahn–Teller parameters. [ABSTRACT FROM AUTHOR]