Characterization of supersonic beams of polyatomic molecules

Supersonic free jets and beams have been characterized in terms of the terminal Mach number, velocity spread, translational temperature and the average value of specific heat ratio (γ) pertinent to supersonic expansion. It has been found that for a polyatomic molecule, the average value of γ, used to describe the flow, depends upon the stagnation conditions. Vibrational relaxation of SF₆ as a representative of polyatomic molecules has been investigated using the “sudden freeze” model of Knuth and the relaxation rate equation model of Quah. Though the “sudden freeze” model predicts the experimental terminal vibrational temperature of Luijks et al., it overestimates cooling close to the nozzle exit. Quah's model shows that the vibrational relaxation cross section (σ_v) increases with decrease in temperature, contrary to the prediction of the Landau—Teller relaxation mechanism. This has been attributed to enhanced efficiency of vibrational relaxation at lower temperature due to facile exchange of internal energy with translational energy through the formation of quasi-resonant and van der Waals complexes.     

Vibrational relaxation induced population inversions in laser pumped polyatomic molecules

Conditions for population inversion in laser pumped polyatomic molecules are described. For systems which exhibit metastable vibrational population distributions [slow vibration—translation/rotation (V—T/R) relaxation], large, long lived inversions are possible even when the vibrational modes are strongly coupled by rapid collisional vibration—vibration (V—V) energy transfer. Overtone states of a hot mode are found to invert with respect to fundamental levels of a cold mode even at V—V steady state. Inversion persists for a V—T/R relaxation time. A gain of 4 m^?1 for the 2v₃ → v₂ transition in CH₃F (λ ≈ 15.9 μ) was found assuming a spontaneous emission lifetime of 10 s for this transition. General equations are derived which can be used to determine the magnitude of population inversion in any laser pumped, vibrationally metastable, polyatomic molecule. A discussion of factors controlling the population maxima of different vibrational states in optically pumped, V—V equilibrated metastable polyatomics is also given.     

Vibrational relaxation of ethylene oxide and ethylene oxide-rare-gas mixtures

The collision-induced vibrational energy relaxation of ethylene oxide (C₂H₄O) was studied by means of laser-induced fluorescence. The time-dependent population of the vibrational modes v₃ and v₅/v₁₂ was measured after excitation of CH-stretching vibrations near 3000 cm^?1. Rate constants for the vibrational energy transfer by collisions with C₂H₄O and the rare gases are deduced, and a simplified model for the vibrational relaxation of C₂H₄O is discussed.     

Mode-to-mode vibrational energy transfer in D2CO: Evidence of coriolis-enhanced rotational selectivity

Infrared—ultraviolet double resonance spectroscopy is used to demonstrate rapid collision-induced V-V transfer between the v₆ and v₄ vibrational manifolds of D₂CO. The rate of transfer is at least gas-kinetic and is explained in terms of Coriolis coupling and rotationally specific, quasi-resonant relaxation channels     

Author index to volume 80

Time-resolved infrared-ultraviolet double-resonance spectroscopy provides a sensitive means of studying collision-induced rotational relaxation rates and mechanisms. Relaxation of the 12_3,10 rotational state in the v₄ = 1 vibrational level of D₂CO is found to follow electric-dipole selection rules with rates determined absolutely to be as much as five times gas kinetic.     

Dependence of the probability of VT-VV coupled transitions on vibrational quantum numbers

The dependence of the probability of v - 1 → v vibrational excitations in molecule-molecule collisions on the vibrational quantum number v has been studied for v up to 7. This nominally VT energy transfer process can be strongly affected by the participation of VV mode particularly among high-lying states. The dependences of the probabilities on v among such states are shown to be non-linear largely due to the VT-VV intermode coupling. A sample calculation of the dependence in iodine molecules shows that the coupling is very strong when the temperature is high and that the probabilities seriously deviate from the linear relation P_v - 1.0 → v.0 = vP_{0.0 → 1.0}, where molecule 1 undergoes transition from v - 1 to v, while molecule 2 remains in the ground state.     

Vibrational energy transfer from C2H4 to CO2 studied at low temperatures in a jet by infrared double resonance

In a mixed CO₂-C₂H₄ jet the populations of rotational levels in the ground andv ₂ (667 cm^?1) vibrational state of CO₂ are probed by color center laser absorption. The rotational distributions are well described by a rotational temperature. On the timescale of the expansion thev ₂ state does only relax through resonant deexcitation by CO₂ clusters. A 10% fraction of C₂H₄ molecules is excited into thev ₇ (949 cm^?1) level by CW CO₂ laser absorption, subsequently releasing energy into the expansion due to relaxation to lower C₂H₄ vibrational states and to thev ₂ mode of CO₂. Observed are the induced rise of rotational temperature and the increase ofv ₂ level population. By probing at several distances from the nozzle the temperature range from 30–155 K is covered. The vibrational transfer of the excited C₂H₄ to thev ₂ level of CO₂ has a near quadratic inverse temperature dependence, increasing by a factor of 40 when the temperature is lowered from 155 K to 30 K.     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

Vibrational kinetics in HeCO reacting discharges

Combined measurements of vibrational distributions (N_υ) of CO and CO₂ yields (β) in HeCO discharges have been performed at different residence times in radiofrequency discharges. The experimental results on N_υ have been obtained by IR emission spectroscopy and on β by gas-chromatographic and mass-spectrometric techniques. A theoretical model including the most important relaxation channels of the vibrational energy has been set up and coupled to the plasma chemistry describing the rate of formation of species such as CO₂, C, and O. Theoretical and experimental results are in good agreement, emphasizing the role of a vibrational mechanism in dissociating CO in HeCO mixtures.     

Radiative lifetime of CO+ (X2∑+, ν) ions

The radiative relaxation of CO⁺(X²∑⁺, ν) ions produced by ionization of CO and of OCS with 70 eV electrons has been investigated using the monitor ion technique in a triple FT ICR spectrometer. Since the monitor reaction is exothermic for ν ≥ 1, the experimental lifetimes, 170 ms for CO⁺ from CO and 210 ms for CO⁺ from OCS, should correspond to the overall decay rate of ν ≥ 2 with a small contribution of ν = 1.These values being much larger than the theoretical lifetime of ν = 1 (128.5 ms), computer-simulated overall lifetimes for ν ≥ 1 and ν ≥ 2 have been determined using theoretical lifetimes calculated by Rosmus and Werner and a vibrational population deduced from photoelectron spectroscopy of CO with 40.8 eV photons (Gardner and Samson). The computer-simulated lifetimes, 250 ms for ν ≥ 1 and 125 ms for ν ≥ 2, are indeed about twice as large as the theoretical lifetimes for ν = 1 and ν = 2, respectively, indicating that the large experimental lifetimes may be accounted for by radiative cascade from the upper vibrational levels.     

Progress in the non-equilibrium vibrational kinetics of hydrogen in magnetic multicusp H− ion sources

The non-equilibrium vibrational kinetics of H₂ in multicusp magnetic discharges has been studied by improving a previous model developed by our groups. In particular, a complete set of V-T (vibrational translation) rates involving H-H₂(v) collisions, calculated by using a three-dimensional dynamics approach, has been inserted into our self-consistent model for better representing the corresponding relaxation. Different experimental situations are simulated with special emphasis on the temporal scales necessary for the different distributions (electron energy and vibrational distributions) to reach stationary values. Finally, a comparison between theoretical and experimental quantities such as vibrational temperature, electron temperature, electron number density and concentration of negative ions (H⁻) shows a satisfactory agreement, thus indicating the basic correctness of our model.     

Vibrational energy disposal in reactions of fluorine atoms with hydrides of groups III,IV and V

The HF infrared chemiluminescence from the reactions of F atoms with B₂H₆, CH₄, CH₃F, CH₂F₂, CH₂Cl₂, CH₃ONO. CH₃NO₂, NH₃ (and ND₃). PH₃ and HNCO has been observed from a 300 K flowing-afterglow reactor. Experiments were done for a range of CH₄ and F atom concentrations to identify conditions which were free of vibrational relaxation and secondary reactions, and these conditions were used to assign initial HF(v) vibrational distributions for each reaction. The emission intensity from each reaction also was compared to that from CH₄ in order to obtain the relative HF formation rate constants at 300 K. Since the absolute rate constant for F + CH₄ is well established, the combination of all of these data provides absolute rate constants for HF(v) formation at 300 K. The ND₃ reaction was studied to obtain information on more vibrational levels in order to better estimate the HF(v = 0) and DF(v = 0) components of the ammonia distributions. With NH₃ and ND₃ there is no significant isotope effect on the energy disposal. Except for NHCO, for which an addition-elimination channel is possible, the HF(v) distributions are inverted and <f_v > = 0.60. Differences between the HF(v) distributions reported here and some other reports in the literature are noted: the present data are discussed as representative of direct H atom abstraction for 300 K Boltzmann conditions. The HCl infrared chemiluminescence from the F + CHCl₂ secondary reaction also was observed; the HCl(v) distribution was v₁: v₂: v₃: v₄: v₅ - 0.47: 0.23: 0.18: 0.08: 0.04.     

Semiclassical calculation of cross sections and rate constants for vibrational deactivation of HD (v = 1) colliding with 4He

A semiclassical collision model has been used to calculate the rate constant for vibrational relaxation in HD (v = 1, j = 0) colliding with ⁴He. The He + HD potential surface was obtained from an analytical He + H₂ surface previously used for similar calculations on He + H₂ and He + D₂. The theoretically calculated rate constant is about 50% below that experimentally determined in the temperature range 80–300 K.     

Vibrational relaxation of CO2(v3) by O atoms

The vibrational relaxation time for CO₂(v₃) + O(³P) has been measured by laser fluorescence. The observed value, βCO₂.O = 0.21 ± 0.04 μsec, is an order of magnitude lower than the relaxation time for self-collisions.     

Chemical lasers produced from O(3P) atom reactions. V. CO laser emissions and vibrational population distribution in the flash-initiated SO2-CFBr3 system

CO laser emission at 5 μm was detected when SO₂ and CFBr₃ were flash photolyzed in the vacuum ultraviolet above 165 nm. Over 40 vibrational–rotational transitions ranging from Δv = 2 → 1 to 14 → 13, with the exception of those between 8 → 7 and 11 → 10, were identified. The CO emission is believed to result from the O + CF reaction: The vibrational population of the CO has been measured by means of a CO laser resonance absorption method. The CO was found to be vibrationally excited to v = 24 with a vibrational temperature of about 1.4 × 10⁴°K. The “surprisal analysis” of the observed CO distribution showed the possible occurrence of a minor process (presumably O + CFBr) that generated vibrationally colder CO. The effects of various additives on the CO emission were also examined. The addition of CO₂ to a D₂-SO₂-CFBr₃-He mixture resulted in a simultaneous osciallation at 3.6, 5, and 10.6 μm due to DF, CO, and CO₂, respectively. Additionally, the utilization of the O + CF_n (n = 1, 2, 3) reactions as F-atom sources for HF-laser operation in flash-initiated systems were demonstrated.     

Infrared spectroscopy and intramolecular vibrational relaxation of c-C6F12 excited above the dissociation threshold

The IR spectrum of c-C₆F₁₂ at a vibrational energy of twice the dissociation threshold was investigated. Absorption of cw CO₂ laser radiation was measured at various frequencies. Our experimental conditions were chosen such that during absorption measurements all vibrational degrees of freedom were in equilibrium, the molecular rotation being at room temperature. The Boltzmann vibrational distribution allowed computer simulations of the spectrum to be made to determine the homogeneous contribution. The homogeneous half-width of the spectrum is γ=13±0.5 cm⁻¹ and the homogeneous spectrum of c-C₆F₁₂ at E= 60000 cm⁻¹ is non-Lorentzian. We attribute this to the influence of higher-order anharmonicities on the relaxation from the excited mode (v₂₇) to other modes in the molecule.     

On the vibrational structure of methyl iodide. II. relaxation pathways in the 3000 cm−1 energy regime

Model calculations for state collision induced vibrational relaxation rates are performed. The model includes intramolecular couplings to fourth order. It is shown that Fermi resonance interactions between the v_i CH-stretching mode and overtones or higher order combination tones are essential to understand the rapid energy relaxation into these states.     

Calculated lifetimes and linewidth temperature dependence (1.5–150 K) of four internal vibrations of naphthalene crystal

The linewidths of four totally symmetric vibrations of naphthalene have been calculated as a function of temperature between 1.5 and 150 K on the basis of a population relaxation mechanism. The anharmonic coefficients coupling internal and lattice phonons are obtained from an intermolecular potential that includes atom-atom and quadrupole-quadrupole contributions. The agreement with the experimental data is good for two of the modes considered, while it is unsatisfactory for the others. The results show that the population relaxation (T₁ decay) can contribute substantially to the observed dephasing also at relatively high temperature. The internal and external modes responsible for the vibrational decay are individualized, showing that several distinct two-phonon processes with different weights are involved in the relaxation of the optical modes.     

Vibrational relaxation and collision-induced dissociation of xenon fluoride by neon

Rate coefficients were calculated for vibrational relaxation and collision-induced dissociation of ground state xenon fluoride in neon at temperatures between 300 and 1000 K for each of nine vibrational levels. These coefficients were calculated using a pairwise additive potential energy surface, which consists of a Morse function for the XeF interaction and Lennard–Jones functions for the NeXe and NeF interactions. Rate coefficients are provided for both temperature and v- dependences. The vibrational relaxation and dissociation processes occur by multiquanta transitions. Dissociation can take place from all v-levels provided that the internal energy of the XeF molecule is close to the rotationless dissociation limit. The order of increase effectiveness of the various forms of energy in promoting dissociation in XeF was found to be translation–rotation-vibration. At room temperature, neon atoms were found to be more efficient than helium atoms in the dissociation processes; helium atoms were found to be more efficient than neon atoms in the vibrational relaxation of XeF. Strong vibration–rotation coupling in both vibrational relaxation and in the dissociation processes is demonstrated.     

Vibrational relaxation by reversible intersystem crossing

The time- and energy-resolved fluorescence spectra of CO obtained under selective excitation of the A ¹Π (υ′ = 1) level have been studied. The apparent vibrational relaxation in the singlet manifold is shown to be mainly due to the three-step process: the singlet → triplet crossing from υ′ = 1 level, vibrational relaxation in the triplet manifold and reverse crossing to the A ¹Π (υ′ = 0) level. The decay form may be fitted by assuming the relaxation rate constants in triplet (and singlet) manifolds of the order of 10⁵ s^?1 Torr^?1 i.e. smaller by one order of magnitude than previously proposed.