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Shaped ultrafast laser pulses were used to study and control the ionization dynamics of electronically excited pyrazine in a pump and probe experiment. For pump pulses created without feedback from the product signal, the ion growth curve (the parent ion signal as a function of pump/probe delay) was described quantitatively by the classical rate equations for internal conversion of the S2 and S1 states. Very different, non-classical behavior was observed when a genetic algorithm (GA) employing phase-only modulationwas used to minimize the ion signal at some pre-determined target time, T. Two qualitatively different control mechanisms were identified for early (T < 1.5 ps) and late (T > 1.5 ps) target times. In the former case, the ion signalwas largely suppressed for t < T,while for t » T, the ion signal produced by the GA-optimized pulse and a transform limited (TL) pulse coalesced. In contrast, for T > 1.5 ps, the ion growth curve followed the classical rate equations for t < T, while for t » T, the quantum yield for the GA-optimized pulse was much smaller than for a TL pulse. We interpret the first type of behavior as an indication that the wave packet produced by the pump laser is localized in a region of the S2 potential energy surface where the vertical ionization energy exceeds the probe photon energy, whereas the second type of behavior may be described by a reduced absorption cross section for S0 → S2 followed by incoherent decay of the excited molecules. Amplitude modulation observed in the spectrum of the shaped pulse may have contributed to the control mechanism, although this possibility is mitigated by the very small focal volume of the probe laser. [ABSTRACT FROM AUTHOR]

The electric field of a light wave accumulates a π phase shift as it passes through a focus. We show here how this effect, known as the Gouy phase, may be used to control the branching ratio of a unimolecular reaction when the products are formed with different numbers of photons. We demonstrate this control method for the ionization and dissociation of vinyl chloride, using absorption of 177 and 532 nm photons to induce a pair of interfering paths. Excellent agreement between the observed and calculated phase shift as a function of the axial coordinate of the laser focus indicates that fragmentation occurs via a ladder switching mechanism. The axial dependence of the modulation depth is evidence of loss of coherence at higher internal temperatures of the molecule. [ABSTRACT FROM AUTHOR]

Pairs of 80 fs, 800 nm laser pulses were used to ablate Si, Cu, and CaF2 in air. The spectrally resolved plasma emission was measured as a function of laser fluence and pulse delay. After an initial dip, the fluorescence was found to increase monotonically with pulse delay, reaching a plateau after some tens of picoseconds, depending on the material and fluence. The enhancement ratio (defined as the ratio of the fluorescence produced by the pulse pair to that produced by a single pulse of the same total fluence) reaches a maximum value of 6 and 11 at a fluence of ∼6 J/cm2 for Si and Cu, respectively, and declines to a value below 2 at higher fluences. In contrast, the enhancement for CaF2 increases slowly from zero near threshold to a broad maximum value of 2 near 50 J/cm2. Using reflectivity and atomic force microscopy measurements as diagnostics, we interpret the Si and Cu behavior in terms of a two phase mechanism, in which the first pulse melts the surface of the crystal and the second pulse ablates the resulting liquid film. A qualitatively different mechanism initiated by multiphoton absorption is involved in CaF2 ablation. [ABSTRACT FROM AUTHOR]

We show how the spatial phase of a focused laser beam may be used as a tool for controlling the branching ratio of a chemical reaction. Guoy discovered [Acad. Sci., Paris, C. R. 110, 1250 (1890)] that when an electromagnetic wave passes through a focus its phase increases by π. In a coherent control scheme involving the absorption of n photons of frequency ωm and m photons of frequency ωn, the overall phase shift produced by the Gouy phase is (n-m)π. At any given point in space, this phase shift is identical for all reaction products. Nevertheless, if the yields for different reaction channels have different intensity dependencies, the Gouy phase produces a net phase lag between the products that varies with the axial coordinate of the laser focus. We obtain here analytical and numerical values of this phase as the laser focus is scanned across the diameter of the molecular beam, taking into account the Rayleigh range and astigmatism of the laser beam and saturation of the transition. We also show that the modulation depth of the interference pattern may be increased by optimizing the relative intensities of the two fields. [ABSTRACT FROM AUTHOR]

We explore the possibility of using an intense laser beam to focus a molecular beam onto a surface to create nanowires. We show that with a grazing angle of incidence between the laser and molecular beams, it is possible to use available technology to create wires < 50 nm wide and > 100 µm long with a 100 W continuous wave laser. Narrower and longer features could be created with higher power lasers. This technique is very general, and may be used to deposit any atom or molecule onto an arbitrary substrate, so long as the particles may be entrained in a molecular beam and have an adequate sticking probability. The effects of spherical and chromatic aberration and laser mode structure on the focusing properties of the molecular lens are examined in detail, and design criteria for building a practical device are discussed. [ABSTRACT FROM AUTHOR]

The speed and angular distributions of I[sup +] ions, produced when IC1 molecules were exposed to both ultraviolet and visible radiation at 304+608 nm, 355+608 nm, and 304+532 nm, were measured by velocity map imaging. An intense central feature in the I[sup +] images was observed to be very sensitive to the polarization of the ultraviolet light and is attributed to a dissociative ionization mechanism involving three-body fragmentation: ICI+ hv (visible) + 3hv (ultraviolet) → I[sup +] + Cl + e[sup -]. The effect of varying the delay between the visible and ultraviolet radiation on the I[sup +] images suggests that an intermediate gateway state of ICl reached by absorption of one photon of visible light mediates the transition to the superexcited dissociative ionization state. [ABSTRACT FROM AUTHOR]

Phase lag spectroscopic measurements are performed for HI and DI in the 5d(π, δ) resonance region. Measurements of the phase lags for both molecules reveals a strong isotopic effect, especially pronounced in the ionization channel. The energy dependence of the phase lag provides clear examples of continuum and resonance coupling effects that were previously predicted theoretically but have not yet been observed. It is demonstrated that phase lag spectroscopy is a sensitive technique for studying properties of molecular continua, furnishing information that is not available from phase-insensitive observables. [ABSTRACT FROM AUTHOR]

Examines the Rydberg electronic states of hydrogen chloride through the deperturbation of Hamiltonian calculations. Description of the deperturbation scheme; Results of hydrogen chloride spectrum deperturbation; Description of intensity anomalies and photodissociation dynamics.

Studies a semiclassical model of the angular distribution of the photofragments of predissociating molecules. Neglect of exit channel effects; Comparison of the model with velocity-aligned Doppler spectra of CI atoms.

We have used the method of coherent phase modulation, first proposed by Brumer and Shapiro for controlling the rates of chemical reactions, to measure the refractive index of a gas in the vacuum ultraviolet. In these experiments we populated a Rydberg state of HCl or CO by simultaneous absorption of one VUV photon and three UV photons. Molecular ions were generated by absorption of an additional UV photon and detected with a time-of-flight mass spectrometer. The phase difference for the UV and VUV beams was altered by passing them through a cell containing either H2 or Ar. By varying the pressure in the cell we could control this phase difference and thereby modulate the ion signal. From the modulation frequency of the signal it was possible to determine the difference between the refractive indices of the gas at the UV and VUV wavelengths. Using reliable refractive indices in the UV, we extracted from our data values for the VUV indices. These values are in quantitative agreement with a calculation that uses dipole oscillator strengths constructed using constrained oscillator strength methods, taking into account the dispersion resulting from individual rotational transitions. The experimental method can be used very close to an optical transition of the refractive medium, provided that the refractive index does not vary appreciably across the bandwidth of the VUV laser. [ABSTRACT FROM AUTHOR]

Amplified spontaneous emission (ASE) is a process which competes with laser-induced fluorescence (LIF) in three level systems. This was demonstrated in the case of oxygen atoms, for which both ASE and LIF were observed at 845 nm. Ground state oxygen atoms were generated by photodissociating O2, SO2, and NO2 in a pump and probe experiment. Both nascent and thermalized O(3Pj‘) atoms were measured using two-photon LIF at 226 nm. The fine structure populations deduced from the 845 nm (3p3Pj’→3s3S) emission were considerably hotter than the distributions obtained from the 130 nm (3s3S→2p3P) transition. A kinetic model is developed showing that the distributions observed at 845 are distorted by ASE, which selectively depletes the populations of atoms originating from more highly populated multiplet levels. [ABSTRACT FROM AUTHOR]

The fine structure population of O(3P) produced in the photodissociation of O2 at 157 nm was measured in a pump-and-probe experiment. The population of the ground state (J=2) was found to be 93%, with only 6% in the J=1 level and 1% in J=0. The predominance of J=2 is consistent with direct dissociation on the B 3Σ-u potential energy surface. Based on available information in the literature we rule out excitation of the 3Πu state and curve crossing to the 5Πu state as possible origins of the population with J<2. We infer that the populations of J<2 is due to nonadiabatic transitions at large distances. Two mechanisms consistent with the data are a Demkov-type interaction, which is caused by coupling by the radial kinetic energy operator. [ABSTRACT FROM AUTHOR]

The technique of resonantly enhanced multiphoton ionization with time-of-flight mass resolution has been used to measure the two-photon absorption spectrum of HBr. We have analyzed 37 bands and assigned them to transitions to the valence, ion-pair state and to Rydberg states belonging to the sσ, pσ, dσ, pπ, dπ, and dδ manifolds. Our results are in excellent agreement with the earlier work of Ginter, Ginter, and Tilford. In addition, we have analyzed twelve new electronic states as well as numerous bands of vibrationally excited levels of previously known electronic states. Strong mixing was observed among the 0+ states. [ABSTRACT FROM AUTHOR]

The vibrational relaxation of SiF4 and C6F5H in an Ar bath was measured as a function of initial energy using the method of time-resolved optoacoustics. SiF4 was found to relax nonexponentially, with a rate constant which varied approximately as the vibrational energy raised to the 3/2 power. This behavior is similar to that observed earlier for SF6+Ar below the bottleneck. In contrast, C6F5H was observed to decay exponentially, as expected for a large molecule in the quasicontinuum. These two types of behavior are discussed in the context of previously proposed propensity rules for the vibrational relaxation of highly excited molecules. [ABSTRACT FROM AUTHOR]

The branching ratio was measured for the production of O(3P) in the photodissociation of CO2 at 157 nm. A gas mixture consisting of CO2, H2, and Ar was irradiated with an F2 excimer laser, while the relative concentration of O(3P) was monitored continuously using atomic resonance fluorescence. The O(1D) product was removed by either reacting with H2 or by being quenched by CO2. At a high H2/CO2 ratio a residual O(3P) signal persisted which was due to the nascent photofragments of CO2. A Stern–Volmer analysis indicated that the fraction of O(3P) produced is 5.9%. Control experiments using O2 and N2O as precursor molecules confirmed this interpretation of the data. A mechanism is proposed based on curve crossing from the 1B2 to the 3B2 potential energy surfaces of CO2. Since the 1B2 state is bent, a substantial fraction of the absorbed energy is initially in bending motion, resulting in a long-lived chaotic trajectory which has many opportunities to cross over to the triplet surface. A similar model proposed by Tully to explain the quenching of O(1D) by CO is in qualitative agreement with our data. [ABSTRACT FROM AUTHOR]

The acoustic waves generated by a relaxing gas have a shape which depends on the ratio of the relaxation rate to the acoustic propagation rate. It is shown how the relaxation time constant can be extracted from this waveform for a variety of relaxation rate laws, using a Green’s function to solve the linearized wave equation. The accuracy of the method is confirmed by measuring the relaxation rate constants for OCS diluted in Ar and He. These, plus a previous measurement of the relaxation rates of highly excited SF6 and C6F5H in Ar, are in good agreement with the literature. [ABSTRACT FROM AUTHOR]

The method of time-resolved optoacoustics was used to measure the time constant τ for the vibrational relaxation of SF6 by Ar as a function of the average initial excitation 〈〈E〉〉0. It was found that pτ=210±30 μs Torr for <>0 between 4000 and 19 000 cm-1, implying nearly pure exponential decay. The average amount of energy change per collision varies linearly with <>, having a value of -8 cm-1/collision at 19 000 cm-1. Below 4000 cm-1 pτ increases, approaching the low energy value obtained from IR fluorescence measurements. [ABSTRACT FROM AUTHOR]

The reaction of O(3P)+H2, D2, and HD was studied by the flash photolysis-resonance fluorescence method. The rate constants kXY(T) were measured over the temperature ranges 297–471 K for XY=H2, 422–472 K for D2, and 422–473 K for HD. Above 400 K kH2 are in excellent agreement with earlier flow measurements, but below this temperature most of the flow data are systematically lower. A small deviation from the Arrhenius function was observed for kH2 at 297 K. Our measurements of kD2 are in excellent agreement with the results of Westenberg and deHaas and differ from those Clyne and Thrush. The measured values of kHD equal the arithmetic mean of kH2 and kD2 within experimental error. A detailed comparison is made between the present results and the CEQB calculations of Bowman, Wagner, Walch, and Dunning and the ICVT/LAG calculations of Garrett and Truhlar. Good overall agreement is obtained with both theories. [ABSTRACT FROM AUTHOR]

A study of the 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of DCl is reported. Transition energies for excitation of the F 1Δ–X 1Σ+ (0,0) and (1,0) bands, as well as the V 1Σ+–X 1Σ+(v′,0) bands, for v′=15–19, are presented. The derived molecular constants for the F–X (0,0) and the V–X bands agree well with those previously obtained from analysis of the one-photon VUV absorption spectrum [A. E. Douglas and F. R. Greening, Can. J. Phys. 57, 1650 (1979)]. The ion signals for excitation through various rotational lines in the E–X (0,0) and F–X (0,0) and (1,0) bands are compared with theoretical two-photon line strengths. Extensive power- and J′-dependent ion fragmentation is observed for the former band. No fragmentation is observed in the F–X bands; however, the ion signal strengths are found to vary strongly with J′. This variation of REMPI signal strengths vs J′ was shown to be due to an indirect predissociation, as in HCl. Tables of experimental line strength factors for the F–X (0,0) and (1,0) bands of HCl and DCl are reported. Finally, the relative REMPI detection sensitivities for HCl and DCl, through their respective F–X (0,0) R(1) lines, are compared. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Coherent phase control of the photodissociation of CH3I has been achieved by quantum mechanical interference between competing paths. The control was accomplished by exciting the parent molecules with three UV photons of frequency ω1 and one VUV photon of frequency ω3=3ω1. Varying the phase difference between the two laser beams resulted in a modulation of the I+ and CH+3 signals, without affecting the parent ion signal. We propose a mechanism in which control occurs over the photodissociation step to produce CH3+I*, followed by ionization of the neutral fragments by additional UV photons. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The fine structure branching ratio of chlorine atoms produced in the photodissociation and predissociation of HCl was measured for excitation energies between 51 800 and 83 800 cm-1. The branching ratio between 60 000 and 70 000 cm-1 is in good agreement with the calculations of Alexander et al. [J. Chem. Phys. 99, 1752 (1993)], converging to the statistical limit at high energy. Predissociation of electronically excited bound states display a surprisingly rich behavior, demonstrating that the nature of the predissociating continuum state has a large effect on the atom fragment branching ratio. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Velocity-aligned Doppler spectroscopy was used to measure the speed distribution functions of HCl(v″=0–2,J″) produced in the 193 nm photodissociation of vinyl chloride. A mechanism which is consistent with our present data as well with our earlier work is three center elimination of HCl followed by concerted but nonsynchronous isomerization of the vinylidene fragment, with some of the isomerization energy converted to kinetic energy of the recoiling fragments. In order to explain the vibrational dependence of the translational and rotational energy distributions, the mechanism is assumed to be vibrationally adiabatic. In addition, magic angle Doppler spectroscopy was used to measure the speed distribution functions of Cl atoms produced in the photodissociation of vinyl chloride and the three dichloroethylene isomers. Bimodal energy distributions were observed for both spin–orbit states, with the Cl(2P1/2) fragment having more kinetic energy than Cl(2P3/2). The data are consistent with competitive reaction on two potential energy surfaces. Adiabatic correlation, with some scrambling in the asymptotic region, provide a qualitative explanation of the spin–orbit branching ratios. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The elimination of H2 in the photodissociation of mono- and di-chloroethylenes was studied with a pump-and-probe technique. A 193 nm excimer laser was used to photodissociate the parent molecules, and a tunable dye laser was used to probe the H2 fragment by 2+1 resonance-enhanced multiphoton ionization (REMPI). The nascent rotational state distributions of H2(X 1Σ+g,v″=0–4) were extracted from the REMPI spectra, and were found to have Boltzmann-type distributions. The maximum and average translational energies for some of the rovibrational levels of H2 were measured using magic angle Doppler spectroscopy. The translational energy of the fragments plus the internal energy of H2 was found to exceed the available energy for a three-center elimination mechanism. It is concluded that a migration mechanism plays a significant role in H2 elimination. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Coherent phase control was demonstrated for a bound-to-continuum transition of a polyatomic molecule. Three UV photons of frequency ω1 and one vacuum ultraviolet (VUV) photon of frequency ω3=3ω1 simultaneously excited H2S above its ionization threshold. The parent ion, H2S+, and fragment ions, HS+ and S+, produced by absorption of additional photons, were observed. All three ion signals were modulated as the phase difference between the light fields was varied. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have used partially deuterated vinyl chloride to elucidate the photodissociation mechanism of this molecule. We have found that 75% of the HCl fragments are produced by three-center α,α elimination. Surprisingly, the rotational energy distributions of HCl are the same for both elimination pathways. One possible explanation is that H atom migration in the parent molecule results in similar transition states for the two pathways. Alternatively, dissociation and fragment isomerization may be strongly coupled so that the product angular momentum distribution is determined late in the reaction coordinate. Very different rotational state distributions were observed for v‘=0 and v‘>=0. We speculate that this dichotomy is caused by formation of a hydrogen-bonded π complex between HCl(v‘>=0) and acetylene. We have also determined that HCl+ ions are produced primarily by an α,β mechanism. This mechanism consists of electronic excitation of vinyl chloride, followed by photoelimination and photoionization of electronically excited HCl*. Finally, we determined that H atoms are preferentially produced by detachment from the β carbon, as predicted from the relative stabilities of the α- and β-chlorovinyl radicals. [ABSTRACT FROM AUTHOR]

Doppler profiles of chlorine and hydrogen atomic fragments produced in the photodissociation of mono- and dichloroethylenes at 193 nm have been measured in a pump-and-probe experiment using 2+1 resonance-enhanced multiphoton ionization. In a second experiment, the angular distributions of the Cl fragments produced from chloroethylenes at 235 and 238 nm were measured using a perfect-focusing mass spectrometer. In a third experiment, we measured the power dependence of the relative yields of H, Cl, HCl, and HCl+ produced from vinyl chloride at 193 nm. For Cl detachment, two primary processes have been confirmed. One produces an isotropic angular distribution of photofragments, while the other produces an anisotropic distribution. For H atom detachment, an isotropic angular distribution and a Boltzmann velocity distribution were found. The ratio of yields of the Cl and H fragments was found to be 4±1 for CH2CCl2 and higher than 10 for t-CHClCHCl and CCl2CClH. The H, Cl, and HCl yields were found to be first order in laser intensity, while the HCl+ yield was found to be third order. Saturation measurements of the ion yield indicate that the latter results from a 1+1+1 resonance-enhanced process involving a bound state of the parent molecule. This intermediate state may also be responsible for producing the statistical component of the Cl atom product. [ABSTRACT FROM AUTHOR]

We have used the method of Shapiro, Hepburn, and Brumer to control the ionization rates of HCl and CO. This method is based on the principle of quantum mechanical interference between two competing paths. In this case the two paths are one- and three-photon excitation of an intermediate Rydberg state. A vacuum ultraviolet (VUV) (110–115 nm) laser beam was produced by third-harmonic generation from an UV beam in Kr gas. The relative phases of the two beams were controlled by passing them through a chamber containing either Ar or H2. The laser beams were focused into a molecular beam, and the ionization products were measured in a time-of-flight mass spectrometer. As the pressure of the phase-tuning gas was varied, the ionization signal was found to oscillate with a period that is proportional to the difference of the indices of refraction of the tuning gas at the UV and VUV wavelengths. We have applied this technique to transitions involving different intermediate electronic states [the j 3Σ-(0+), H 1Σ+(0+), and m 3Π(1) states of HCl and the B 1Σ+ state of CO], different rotational lines of the same electronic transition, and different competing processes (ionization vs dissociation) following excitation of the intermediate state. [ABSTRACT FROM AUTHOR]

The O(2p 3Pj) (j=2, 1, and 0) fragments produced in the 157 nm photodissociation of CO2 were detected by resonance-enhanced multiphoton ionization in a molecular beam. The Doppler profiles and fine-structure branching ratios were measured for the oxygen-atom photofragment in the 3Pj states. The Doppler profiles were analyzed to give an anisotropy parameter of β=2.0±0.2 and an internal energy equivalent to 3.9±0.3 vibrational quanta of CO. The fine-structure populations were found to be 0.70±0.05, 0.16±0.03, and 0.14±0.03 (with error bars of ±σ) for j=2, 1, and 0, respectively. A mechanism is proposed in which complex on the 1B2 surface undergoes intersystem crossing to the 3B2 surface. A phase-space model with a constraint on the impact parameter is shown to be consistent with the observed energy release. The nonstatistical fine-structure population could be caused by long-range interactions on the triplet surface. In a bulb experiment, O(3P) was produced by quenching of O(1D). The fine-structure populations of the resulting O(3Pj) were 0.64±0.04, 0.25±0.04, and 0.11±0.04. This state distribution is consistent with a long-lived complex which decays to give statistical products. [ABSTRACT FROM AUTHOR]

The 2+1 REMPI spectra of the F(0–0), F(1–0), E(0–0), and V(11–0) transitions of HCl were measured in a time-of-flight molecular beam machine. Both room temperature HCl, and rotationally and vibrationally excited HCl obtained by photodissociating vinyl chloride at 193 nm, were used. Several anomalies in the F state were observed. These include a decrease in intensity of the P, Q, and R branches between J’=3 and J’=10 for the 0–0 transition, a falloff in intensity for J’>5 for the P, Q, and R branches of the 1–0 transition, and an enhanced intensity loss of the Q(9) line of the 0–0 transition. Accurate values of the lambda-doublet splitting constants for these transitions were also obtained. The intensity loss in the 0–0 transition is due to an interaction with some unobserved bound state, which could also explain the 1–0 perturbation. A candidate for the perturbing state is the e 3Σ+(1). The isolated perturbation of the Q(9), 0–0 line could be caused by an interaction with the g(0+) state. [ABSTRACT FROM AUTHOR]

The branching ratio for the reaction O(3P)+HD to produce OH and OD was measured over the temperature range 339–500 K using a discharge-flow reactor. The OH and OD products were detected using laser-induced fluorescence under steady-state conditions. In order to determine the relative concentrations of OH and OD produced in this reaction, calibration runs were performed using the reactions of O(3P)+H2 and D2 to produce known relative amounts of the same species. Kinetic modeling showed that secondary reactions and the production of vibrationally excited OD did not affect the measured branching ratio. We found that the OH/OD ratio increased rapidly with decreasing temperature, in qualitative agreement with theory, showing that the reaction is dominated by tunneling below 400 K. At the lowest temperatures, the observed branching ratio is larger than predicted. The reason for this discrepancy is not known. [ABSTRACT FROM AUTHOR]

The rate constant for the reaction of O(3P)+D2 was measured over the temperature range 343 to 446 K using the laser photolysis–resonance fluorescence technique. The results can be fit well with a two-parameter Arrhenius function and are in excellent agreement with the previous measurements of Presser and Gordon and with the recent high temperature measurements of Michael. The data are also in very good agreement with the theoretical calculations of Joseph et al. and of Bowman et al. and demonstrate that tunneling makes a significant contribution to the rate coefficient over the entire range of temperatures studied in this experiment. The Arrhenius behavior of the rate constant supports the conclusions of Robie et al. in their study of the intramolecular isotope effect in O(3P)+HD, where it was found that tunneling plays a major role at low temperatures. [ABSTRACT FROM AUTHOR]

The two-photon spectra of HCl and DCl were studied using the technique of resonantly enhanced multiphoton ionization. Rovibrational bands were observed for the X 1Σ+→V 1Σ+, E 1Σ+, F 1Δ2, d 3Π1, f 3Δ2, and g 3Σ- transitions. The results are compared with the single photon spectra of Ginter, Douglas, and co-workers and with the molecular orbital calculations of Bettendorf et al. [ABSTRACT FROM AUTHOR]

Vibrational energy transfer from SF6 to N2O was studied by using a pulsed CO2 laser for multiphoton excitation of SF6 and monitoring the 4.5 μm emission from N2O(v3=1). The initial average energy of SF6 was selected by varying the fluence of the laser. The vibrational temperature of the ν2 mode of N2O was determined by measuring the attenuation of the fluorescence with a cold gas filter. The vibrational temperature of the ν3 mode was estimated by equating the SF6 and N2O(ν3) vibrational temperatures at the fluorescence maximum. The principal finding of this study is that N2O(ν2) is preferentially excited. While the mode specificity of the VV transfer declines with energy, it is greater than expected statistically. We also observed that the relaxation of ν2 is an order of magnitude slower than the relaxation of ν3. A kinetic model which accounts for most of the observations shows that VV transfer from SF6 to N2O occurs from high lying states of SF6 while the decay of N2O occurs by back transfer to the discrete levels of SF6. The slow relaxation of the bending mode of N2O indicates the existence of a bottleneck in VV transfer between N2O(ν2) and the discrete levels of SF6, which may be caused by a mechanism involving long-range attractive forces. [ABSTRACT FROM AUTHOR]

Studies the partial pressure of vibrationally excited hydrogen molecules in a flowing gas using the technique of vacuum ultraviolet laser-induced fluorescence. Effects of vibrational energy on molecular reaction dynamics; Details on the experiment; Discussion on the results of the study.