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We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si3 was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm-1 region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 13A1″ - ã 3A2′ transition of the D3h isomer. The spectrum has an origin transition at 18 600± 4 cm-1 and a short progression in the symmetric stretch with a frequency of ∼445 cm-1, in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 13A1″ state, which has a calculated symmetric stretching frequency of 480 cm-1. In addition, a ∼505 cm-1 ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D3h state and with theory. A weaker, overlapping band system with a ∼360 cm-1 progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C2v ground state (1A1) state of Si3. Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed. [ABSTRACT FROM AUTHOR]

Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 μs. Hole-burning experiments on the 2 ² Σ u-← X ³ Σg- transition of C6, C8, and ¹Σu+←X ¹Σg+ of C9 confirm the predissociative nature of the excited electronic states. [ABSTRACT FROM AUTHOR]

Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 μs. Hole-burning experiments on the 2 ² Σ u-← X ³ Σg- transition of C6, C8, and ¹Σu+←X ¹Σg+ of C9 confirm the predissociative nature of the excited electronic states. [ABSTRACT FROM AUTHOR]

The electronic spectra of C18 and C22 in the 15 150–36 900 cm-1 range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D9h symmetry for C18 and D11h for C22, based on time-dependent-density-functional calculations and reactivity with respect to H2. The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the Ã1A2″←X1A1′ transition of C18 at 592.89 nm shows a pair of 1 cm-1 broadbands spaced by 1.5 cm-1. The next electronic transitions exhibited much broader, ∼30 (in the visible) to 200 cm-1 (in ultraviolet range), features. The spectrum of C22 exhibits an absorption pattern similar to C18, except that the narrow features to the red are missing; the oscillator strength of the Ã←X transition is predicted to be low. [ABSTRACT FROM AUTHOR]

The visible electronic spectra of the linear l-C[SUB2n+1] H (n=2-4) radicals have been measured in the gas phase. These have been obtained by means of a mass-selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The observed spectra are assigned to the A [SUP2]Δ←X[SUP2]Π, B [SUP2]Σ[SUP-]←X[SUP2]Π, and C [SUP2]Σ[SUP+]←X [SUP2]Π electronic transitions arising from σ→π electron promotion. The assignments are based on ab initio calculations, wavelength dependence of the 0[SUP0,SUB0] transition on size, and isotopic substitution. The lifetime broadening of the bands and effects due to vibronic coupling are associated with the carbon skeleton bending modes. The detection of these carbon chains in the diffuse interstellar medium appears to be more favorable by radio astronomy rather than by electronic spectroscopy. [ABSTRACT FROM AUTHOR]

The rotationally resolved A²II[sub u]+←Χ²II[sub g] electronic origin band spectrum of dicyanodiacetylene cation, NC[sub 6]N[sup +], has been recorded in the gas phase using frequency-production double modulation spectroscopy in a liquid nitrogen cooled hollow cathode discharge and cavity ring down spectroscopy in a supersonic plasma. The analysis of the complementary results provides accurate molecular parameters for the two spin-orbit components in both electronic states. [ABSTRACT FROM AUTHOR]

The A 2Π←X 2Π laser-excitation spectra of chloroacetylene and deuterochloroacetylene cations are reported. Analysis of the band systems yields values for all three stretching fundamentals (ν1–ν3) in the excited electronic state. Rotationally resolved spectra of the 000Ω =3/2 subbands have been recorded and analyzed. The derived rotational constants allow estimates of the H–Cl distances in both electronic states to be made. [ABSTRACT FROM AUTHOR]

Mid-infrared vibrational spectra of He–HN+2 and He2–HN+2 have been recorded by monitoring their photofragmentation in a tandem mass spectrometer. For He–HN+2 three rotationally resolved bands are seen: the fundamental ν1 transition (N–H stretch) at 3158.419±0.009 cm-1, the ν1+νb combination band (N–H stretch plus intermolecular bend) at 3254.671±0.050 cm-1, and the ν1+νs combination band (N–H stretch plus intermolecular stretch) at 3321.466±0.050 cm-1. The spectroscopic data facilitate the development of approximate one-dimensional radial intermolecular potentials relevant to the collinear bonding of He to HN+2 in its (000) and (100) vibrational states. These consist of a short range potential derived from an RKR inversion of the spectroscopic data, together with a long range polarization potential generated by considering the interaction between the He atom and a set of multipoles distributed on the HN+2 nuclei. The following estimates for binding energies are obtained: D0″=378 cm-1 [He+HN+2(000)], and D0′=431 cm-1 [He+HN+2(100)]. While the ν1 band of He2–HN+2 is not rotationally resolved, the fact that it is barely shifted from the corresponding band of He–HN+2 suggests that the trimer possesses a structure in which one of the He atoms occupies a linear proton-bound position forming a He–HN+2 core, to which a second less strongly bound He is attached. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the structural properties of the H2–HCO+ ion-neutral complex has been undertaken. Infrared vibrational predissociation spectra of mass selected H2–HCO+ complexes in the 2500–4200 cm-1 range display several vibrational bands, the most intense arising from excitation of the C–H and H2 stretch vibrations. The latter exhibits resolved rotational structure, being composed of Σ–Σ and Π–Π subbands as expected for a parallel transition of complex with a T-shaped minimum energy geometry. The determined ground state molecular constants are in good agreement with ones obtained by ab initio calculations conducted at the QCISD(T)/6–311G(2df,2pd) level. The complex is composed of largely undistorted H2 and HCO+ subunits, has a T-shaped minimum energy geometry with an H2...HCO+ intermolecular bondlength of approximately 1.75 Å. Broadening of the higher J lines in the P and R branches of the Π–Π subband is proposed to be due to asymmetry type doubling, the magnitude of which is consistent with the calculated barrier to H2 internal rotation. The lower J lines in the Σ–Σ and Π–Π subbands have widths of 0.06 cm-1, around three times larger than the laser bandwidth, corresponding to a decay time of ≊90 ps for the upper level. The absence of discernible rotational structure in the ν2 band suggests that it has predissociation lifetime of less than 1 ps. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The 9–0 and 9–1 transitions of the A 1Πu–X 1Σ+g system of N2+2 have been observed by fast ion beam photofragment spectroscopy. Multireference configuration interaction ab initio calculations predict v’=8 to be the highest bound level which seemed to be supported by experimental evidence. The dominant predissociation mechanism for the v’=9 level appears to be through tunneling, unlike the v’=6, 7, and 8 levels, which show an e/f symmetry-dependent electronic predissociation. Calculated tunneling lifetimes from our own modified A 1Πu state potential gives good agreement with the experimental lifetimes. In addition to the 9–1 and 9–0 bands, we have also measured the previously unobserved 7–0 and 8–0 rovibronic transitions thus extending Cosby, Müller, and Helm’s study to the blue [Phys. Rev. A 28, 766 (1983)]. Molecular constants have been determined for the v=0 of the X 1Σ+g state and v=7, 8, and 9 of the A 1Πu state. [ABSTRACT FROM AUTHOR]

Spectra of 14N+2–20Ne, 14N+2–22Ne, and 15N+2–20Ne have been recorded in the region of the B 2Σ+u←X 2Σ+g origin transition of N+2. Measurements are made by mass selecting cooled ionic complexes and photodissociating them whilst monitoring the N+2 fragment ion intensity as the laser wavelength is scanned. Various bands are assigned to transitions involving the stretching and bending motions of the Ne...Ne+2 bond with their structure and spacings consistent with transitions between quasilinear geometries in the X and the B states. Spectra of complexes with up to eight neon atoms attached to a 14N+2 core have also been measured. Evidence from shifts of the band origins and analysis of the vibrational frequencies of N+2–Ne2 and N+2–Ne3 suggest a structure where the Ne ligands are sited at one end of the N+2 chromophore. [ABSTRACT FROM AUTHOR]

The rotationally resolved origin band in the 2 2E′←X 2A1′ electronic spectrum of cyclic B3 has been observed by cavity ring down spectroscopy in the gas phase. The B3 molecule was generated in a supersonic planar plasma containing decaborane (B10H14) and neon as a carrier gas. The rotational structure pattern is that of a cyclic molecule. It is analyzed assuming an equilateral triangle in both electronic states. The band origin is determined to be 21 853.52 cm-1, and the bond lengths 1.603 77(106) Å in the ground and 1.619 07(96) Å in the excited electronic state are inferred from analysis of the rotational structure. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electronic transitions of the triplet chain C5H- anion were studied using detachment spectroscopy. The system detected in the vicinity of the electron detachment threshold is assigned to the b3A ← a3A transition with a dipole bound state (DBS) character. The second system measured by autodetachment spectroscopy is attributed to the c3A ← a3A Feshbach electronic transition. Negative anharmonicity of the vibrational progression built upon the low frequency CCC in-plane bending mode ν12 is observed in both DBS and Feshbach states. This indicates a barrier to linearity on the potential energy surface in both excited states. The triplet chain C5H- anion exhibits similar electronic properties to C3H-. Renner-Teller and vibronic effects between the 3A DBS and Feshbach states are inferred and may explain the stabilisation of the DBS. This interaction is weaker in C5H- than C3H- leading to smaller barrier heights on the potential energy surface. [ABSTRACT FROM AUTHOR]

Gas phase electronic spectra allow direct comparisons between laboratory data and the wealth of astrophysical measurements that have been tabulated. Over the past decade a number of assorted carbon chain systems have been spectroscopically investigated, yielding an understanding of the transitions and trends apparent among their homologous series. This rigorous program begins with trapping unstable carbon chain ions and radicals in neon matrices, where origin bands and vibrational data are obtained. From these results one can then seek out the gas phase transitions in higher resolution in order to more accurately determine assignments and spectroscopic constants. To this purpose, laser spectroscopic methods are employed, including pulsed and cw cavity ringdown, resonant enhanced multiphoton ionisation, and more recently, a two-photon two-colour excitation/fragmentation approach to investigate trapped ions. From a comparison involving the electronic spectra of a number of carbon chain systems measured in the laboratory with diffuse interstellar band (DIB) absorptions, one concludes that species involving only a handful of carbon atoms can not be the carriers. However, larger carbon chains and ring species still remain viable. In this review, we consider which systems require further investigation, including these longer species, metal-capped chains and other cationic radicals. [ABSTRACT FROM AUTHOR]

Gas-phase electronic spectra of the BNB radical and its isotopomers have been measured in a supersonic molecular beam in the visible spectral region. A mass-selective resonant two-colour two-photon ionization technique coupled to a laser ablation source was employed. Ab initio calculations on the geometries, energies and vertical electronic excitations have been carried out using the Becke 3-parameter Lee-Yang-Parr (B3LYP) method, the spin restricted coupled cluster (RCCSD(T)) approach and the multireference configuration-interaction (MRCI) theory for the linear BNB, BBN, and cyclic B2N isomers. The spectra are assigned as the [image omitted] electronic transition of the linear BNB radical. The vibronic band system is complicated by spin-orbit interaction and the Renner-Teller effect in the [image omitted] 2Πg state. The vibronic structure of the transition has been analysed using the Pople-Hougen theoretical model and the calculated potential curves along the bending coordinate. The resulting spectroscopic constants are [image omitted] = 96 cm-1, 2 [image omitted] = 202 cm-1; [image omitted] = 846 cm-1, [image omitted] = 322 cm-1, ε' = +0.45 and 2 [image omitted] = 1201 cm-1 for 11B14N11B. [ABSTRACT FROM AUTHOR]

The restricted coupled cluster with perturbative triples (RCCSD(T)) method has been used to calculate equilibrium geometries and excitation energies of the hitherto unknown quartet states in diacetylene, triacetylene and benzene radical cations. Spectroscopic data for the doublet states obtained with the same approach are found to be in good agreement with experiment. Whereas the 2Πu and 2Πg states in diacetylene and triacetylene cations form linear-linear Renner-Teller pairs, strongly bent minima are calculated for the quartet states correlating with the 4Πg and 4Πu linear structures. In both ions there are quartets that lie close to the minimum energies of the first electronically excited doublet states. The 4Au state of benzene radical cation lies below C2E1u and could be populated during the ultrafast nonradiative decay of this doublet state. [ABSTRACT FROM AUTHOR]

A high-resolution gas-phase electronic spectrum has been detected in the 408  nm region using a cw cavity ring-down spectrometer. The absorption band system is observed using a pulsed slit nozzle discharge with a mixture of acetylene in argon. The rotationally resolved spectrum obtained is that of a symmetric top molecule. The determined constants are ν  =  24491.0500 (108) and B ′′  =  0.14165 (44)  cm -1 . Consideration of the latter value and ab initio calculations leads to the assignment 3 E  ←   X 3 E transition of the linear carbon chain species in the triplet ground electronic state with C 3v symmetry. [ABSTRACT FROM AUTHOR]

The electronic spectra of the C 3 H - and C 3 D - anions have been studied above the lowest electron detachment threshold. On the basis of the vibrational, rotational analysis and ab initio calculations, the photodetachment spectrum is assigned to the d 3 A″←a 3 A″ Feshbach resonance in the bent chain C 3 H(D) - anion. The vibronic system is characterized by a long vibrational progression involving the CCH in plane bending mode ν 4 . The potential curves along this coordinate obtained from the spectral analysis and theoretical calculations reveal the importance of vibronic coupling in the electronic excited states. A strong Renner-Teller effect is thought to be the reason for the existence of the Feshbach resonance because the 4 Σ - neutral parent and the 3 Π anion excited states are close in energy. As for the neutral, ν 4 appears to be the active mode and drives the interaction between the Feshbach and the dipole bound states. [ABSTRACT FROM AUTHOR]

Rotationally resolved, vibrational predissociation spectra of the HN+2–He complex have been recorded in the region of the N–H stretch (3100–3200 cm-1). The complex appears to be linear. Fitting of the measured lines to the pseudodiatomic expression ν=ν0+(B’ν+B‘ν)m +(B’ν-B‘ν–D’ν+D’ν) m2-2(D’ν+D‘ν)m3 -(D’ν-D‘ν)m4 yields the following constants: ν0=3158.419±0.009 cm-1, B‘=0.3517±0.0005 cm-1, D‘=(5.8±0.5)×106 cm-1, B’=0.3579 ±0.0005 cm-1, D’=(3.9±0.6)×106 cm-1. The data support a proton bound He–HNN+ structure, with a 1.72 Å vibrationally averaged intermolecular bondlength, and an approximate intermolecular stretching frequency of 150 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The origin band in the electronic transition to the dipole bound excited state of C5H(D)¯ anions was measured using two-color photodetachment spectroscopy. The rotational analysis of the partially resolved contour is consistent with a linear structure of the anion in both the ground X3∑¯ and excited A3∏ electronic states, in contrast with an earlier interpretation. The following spectroscopic constants are inferred for C5H¯: T00 = 19248.0(1), B′ = 0.0835(1), B″ = 0.0826(2), A′SO = -11.96(1), λSS′ = 1.97(8)λSS″ =0.24(15). Ab initio calculations at the UHF-MP2 level support the conclusion that C5H¯ is linear in the ground state. The experimentally determined ground state rotational constant can be used in the search for the millimeter wave spectrum of C5H¯. [ABSTRACT FROM AUTHOR]

Theelectronic spectrum of C17H12N+,phenanthrene with a side chain, was measured in the gas phase ata vibrational and rotational temperature of ∼15 K in an iontrap using a resonant multiphoton dissociation technique. The C17H12N+structure was produced in a chemicalionization source and identified by a comparison with theoreticalcalculations of stable structures and excitation energies. The (3), (2), (1) 1A ← X 1A electronic transitions of this nitrogen-containing aromatic specieswith 30 atoms have origin band maxima at 23 586 ± 1 cm–1, 16 120 ± 50 cm–1,and 14 519 ± 30 cm–1. Distinct vibrational structure in the (3) 1A state is observed, and assignments are made. Astronomical aspectsare considered. [ABSTRACT FROM AUTHOR]