Quantum state-resolved collision relaxation of highly vibrationally excited SO2

Collision depopulation cross sections of 13 single, highly vibrationally excited levels with 45,000 cm(-1) energy in the electronic ground state of SO(2) in collision with CO in a supersonic jet have been measured. The measurements for these single highly excited quantum states are conducted through pressure dependence of the decay of the fluorescence quantum beat resulted from their coupling with the rovibronic levels in the optically allowed transitions to the (140), (210), and (132) C(1)B(2) levels. The relaxation cross sections of these highly excited states, each with well-defined energy and symmetry, range from 27 to 187 A(2) with an average of 71 A(2). This average cross section is much larger than the hard sphere cross section of 48 A(2). The relaxation cross section is also found to be larger for the quantum states with a larger matrix element in coupling with the "bright" electronically excited level. Both observations suggest a substantial contribution from long range interactions in collision relaxation of highly excited molecules.     

Generation and characterization of highly vibrationally excited molecular beam

A simple method to generate and characterize a pure highly vibrationally excited azulene molecular beam is demonstrated. Azulene molecules initially excited to the S4 state by 266-nm UV photons reach high vibrationally excited levels of the ground electronic state upon rapid internal conversion from the S4 electronically excited state. VUV laser beams at 157 and 118 nm, respectively, are used to characterize the relative concentrations of the highly vibrationally excited azulene and the rotationally and vibrationally cooled azulene in the molecular beam. With a laser intensity of 34 mJ/cm2, 75% of azulene molecules absorb a single 266-nm photon and become highly vibrationally excited molecules. The remaining ground-state azulene molecules absorb two or more UV photons, ending up either as molecular cations, which are repelled out of the beam by an electric field, or as dissociation fragments, which veer off the molecular-beam axis. No azulene without absorption of UV photons is left in the molecular beam. The molecular beam that contains only highly vibrationally excited molecules and carrier gas is useful in various experiments related to the studies of highly vibrationally excited molecules.     

Solution phase isomerization of vibrationally excited singlet nitrenes to vibrationally excited 1,2-didehydroazepine

Photolysis of phenyl and o-biphenylyl azide (at 270 nm) releases vibrationally excited singlet nitrene which isomerizes to the corresponding hot 1,2-didehydroazepine at a rate competitive with thermal relaxation. Using ultrafast vibrational spectroscopy we observe the formation of vibrationally excited 1,2-4,6-azacycloheptatetraene (1,2-didehydroazepine) in picoseconds following photolysis of phenyl azide in chloroform and o-biphenylyl azide in acetonitrile at ambient temperature.     

Diffusion of vibrationally excited molecules

A treatment is presented for the effect of intermolecular vibrational energy transfer on the diffusion coefficient of vibrationally excited molecules. An analytic treatment based on random walk statistics and a Monte Carlo type calculation have been performed. Both methods yield very similar results which correlate well with existing experimental studies. A hard sphere collision model is treated extensively with comparisons made to other internmolecular potentials. The results support the involvement of long range energy transfer in V → V interactions. The effect of temeprature on the diffusion coefficient of vibrationally excited molecules is calculated, with applications to the CO^*₂CO₂ system.     

Relaxation of highly vibrationally excited molecules

The rate constant for V-V relaxation of diatomic homonuclear molecules is determined from collisions of unexcited molecules with molecules near the dissociation threshold. It is shown that a quasi-resonant transition through several levels dominates in this process so that the energy difference between the initial and final states of the excited molecule is approximately equal to the transition energy from the zero level to the first one. The relaxation kinetics of excited molecules is studied. Absorption of IR radiation by polyatomic molecules is discussed taking into account collisions. A criterion for the negligibility of energy loss is obtained, and the dissociation rate of molecules under the action of IR laser irradiation found. The computational results are compared with experimental data. A self-consistent procedure is formulated for a gas irradiated by a quasi-continuously operating IR laser, in order to determine simultaneously the dissociation rate, dissipation energy flux and temperature. The existence of an optimum radiation region for dissociation is found.     

The decomposition of vibrationally excited molecules

Possible decomposition routes for vibrationally excited hydrocarbons are critically considered using a simple “symmetry” based model. It is shown that C---C bond rupture is characterised by a lower activation energy than the molecular cleavage of hydrogen, although the latter reaction is less endothermic. Other possible decomposition reactions of excited species are also considered.     

Internal conversion from vibrationally excited levels

It is shown that internal conversion to the ground state can become an important pathway of radiationless decay, for molecules in excited vibrational l     

Transient absorption of vibrationally excited ice Ih

The ultrafast dynamics of HDO:D2O ice Ih at 180 K is studied by midinfrared ultrafast pump-probe spectroscopy. The vibrational relaxation of HDO:D2O ice is observed to proceed via an intermediate state, which has a blueshifted absorption spectrum. Polarization resolved measurements reveal that the intermediate state is part of the intramolecular relaxation pathway of the HDO molecule. In addition, slow dynamics on a time scale of the order of 10-100 ps is observed, related to thermally induced collective reorganizations of the ice lattice. The transient absorption line shape is analyzed within a Lippincott-Schroeder model for the OH-stretch potential. This analysis identifies the main mechanism behind the strong spectral broadening of the v(OH)=1-->2 transition.     

Energy transfer of highly vibrationally excited biphenyl

The energy transfer between Kr atoms and highly vibrationally excited, rotationally cold biphenyl in the triplet state was investigated using crossed-beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques. Compared to the energy transfer of naphthalene, energy transfer of biphenyl shows more forward scattering, less complex formation, larger cross section for vibrational to translational (V→T) energy transfer, smaller cross section for translational to vibrational and rotational (T→VR) energy transfer, larger total collisional cross section, and more energy transferred from vibration to translation. Significant increase in the large V→T energy transfer probabilities, termed supercollisions, was observed. The difference in the energy transfer of highly vibrationally excited molecules between rotationally cold naphthalene and rotationally cold biphenyl is very similar to the difference in the energy transfer of highly vibrationally excited molecules between rotationally cold naphthalene and rotationally hot naphthalene. The low-frequency vibrational modes with out-of-plane motion and rotationlike wide-angle motion are attributed to make the energy transfer of biphenyl different from that of naphthalene.     

Reaction of vibrationally excited HBr and I

The rate of the reaction HBr(ν) + I → HI + Br increases by at least a factor of 10⁹ when HBr(ν=0) is excited to HBr(ν 2). The increase is observed when the reacting level is directly excited by sequential laser excitation or when it is populated by collisional excitation. Bromine isotopic enrichment in BrI, the product of a subsequent reaction, was observed after isotopically selective excitation.     

Velocity modulation laser spectroscopy of vibrationally excited CF+ determination of the molecular potential function

The lowest six vibrational hot bands of CF⁺ have been measured in a helium/C₂F₆ discharge by velocity modulation laser spectroscopy. A total of 56 transitions has been fitted to Dunham expansion for v = 0–7, yielding the parameters: ω_e = 1792.6654(18) cm⁻¹B_e = 1.7204176(75) cm⁻¹, Y₂₀, = −13.22968(54) cm⁻¹, and D₀ = 62086(30) cm⁻¹. The rotational temperature of CF⁺ in the plasma is near 650 K and the vibrational temperature is approximately 5200 K.     

Non-radiative decay rate of vibrationally excited triplet molecules

The non-radiative decay rates of triplet benzene and 2-chloronaphthalene were determined as a function of excitation energy. As the excitation energies were increased, the non-radiative decay rates increased gradually. In the case of 2-chloronaphthalene it increased rapidly for the excitation energies above about 38000 cm^-1.     

Collisional deactivation of highly vibrationally excited hexafluorobenzene molecules

The collisional deactivation of the internal energy of vibrationally highly excited hexafluorobenzene (HFB) molecules was examined by the analysis of ultraviolet absorption spectra of excited HFB molecules produced by excitation with an ArF(193 nm) laser. The decay time profile of the internal energy was calculated from the observed absorption decay profile of the hot molecule using the conversion relation between the absorbance by hot molecules and the internal energy. Thus the average energy 〈ΔE〉 transferred per collision was estimated by two different models; energy-independent and energy-dependent function for the decay of the internal energy. The obtained values of 〈ΔE〉 indicate that the energy-dependent model may give reasonable values for 〈ΔE〉, but as far as the value of 〈ΔE〉 is concerned, the energy-independent model is likely to be applicable to the analysis in this reaction system. The collisional deactivation mechanism of the hot HFB molecule and the heating-up effect observed at shorter wavelengths are discussed on the basis of the conversion curve.     

Energy transfer processes during the scattering of vibrationally excited no molecules from a graphite surface

Vibrationally excited NO molecules scattered from a cleaved graphite surface show rotational state populations and rotational accommodation identical to those of ground-state incoming molecules. The survival probability of the vibrational state is found to vary between 75 and 95%, depending on the surface temperature. Transfers of vibrational energy to both molecular rotation and translation are shown to be of only minor importance for the vibrational relaxation during the NO/graphite interaction.     

Rotational spectra of vibrationally excited CCH and CCD

The millimeter-wave rotational spectra of the lowest bending and stretching vibrational levels of CCH and CCD were observed in a low pressure discharge through acetylene and helium. The rotational, centrifugal distortion, and fine structure constants were determined for the (02(0)0) and (02(2)0) bending states, the (100) and (001) stretching levels, and the (011) combination level of CCH. The same pure bending and stretching levels, and the (110) combination level were observed in CCD. Apparent anomalies in the spectroscopic constants in the bending states were shown to be due to l-type resonances. Hyperfine constants, which in CCH are sensitive to the degree of admixture of the A 2Pi excited electronic state, were determined in the excited vibrational levels of both isotopic species. Theoretical Fermi contact and dipole-dipole hyperfine constants calculated by Peric et al. [J. Mol. Spectrosc. 150, 70 (1991)] were found to be in excellent agreement with the measured constants. In CCD, new rotational lines tentatively assigned to the (100) level largely on the basis of the observed hyperfine structure support the assignment of the C-H stretching fundamental (nu1) by Stephens et al. [J. Mol. Struct. 190, 41 (1988)]. Rotational lines in the excited vibrational levels of CCH are fairly intense in our discharge source because the vibrational excitation temperatures of the bending vibrational levels and the (110) and (011) combination levels are only about 100 K higher than the gas kinetic temperature, unlike the higher frequency stretching vibrations, where the excitation temperatures are five to ten times higher.     

Communication: mode-specific photodissociation of vibrationally excited pyrrole

Laser-based spectroscopies coupled with molecular beam techniques facilitated the monitoring of H fragments released in ultraviolet photodissociation of pre-excited isoenergetic vibrational levels of pyrrole. Most noticeably, there was an order of magnitude larger reactivity for an eigenstate primarily consisting of two quanta of ring deformation than for another with one quantum of symmetric C-H stretch. The dynamics, the intramolecular interactions controlling the energy flow, and the mode-selectivity within a medium-sized, ten atom molecule, is discussed.     

Relaxation of vibrationally excited HCl in solid Xe

Rates for vibrational relaxation of HCl(ν = 1.2) in solid xenon at 40 and 146 K are reported and are compared to the rate of relaxation of HCl(ν = 1) in liquid xenon near the freezing point. Upon freezing, the rate of relaxation of HCl(ν = 1) is found to decrease significantly and emission from HCl(ν = 2), absent in the liquid phase, is detected. Both of these effects are attributed to a significant decrease in mobility of HCl molecules in the solid phase as compared to the liquid phase. At both 40 and 146 K, the ratio of relaxation rates for HCl(ν = 2) to HCl(ν = 1) is found to deviate significantly from the harmonic oscillator prediction of 2:1. The rate of relaxation for HCl(ν = 1) by xenon is found to be similar in both liquid solution at 200 K and in the solid at 146 K.     

Cooperative effects in photon statistics of molecular dimers with spectral diffusion

The two-point fluorescence intensity correlation function g(2)t and the Mandel parameter Mt are calculated for a strongly pumped dimer of two-level molecules undergoing Gaussian-Markovian frequency fluctuations. The effects of detuning and saturation are examined. All fluctuation time scale regimes are explored using a continued fraction solution of the stochastic Liouville equation for the generating function. Bunching and antibunching are observed for slow and fast fluctuations, respectively. The short-time antibunching dip in g(2) and its variation with intermolecular coupling, the exciton annihilation rate, and laser detuning are studied.     

Intermode energy transfer in vibrationally excited O3

The laser excited fluorescence method has been employed to determine the rate constants for vibrational relaxation of the O₃ (010), O₃ (100) and O₃ (001) levels at 298 K. The fluorescence observations from the O₃ (010) level provide direct measurements of the rate for intermode vibrational energy transfer from the coupled ν₁ and ν₃ modes to the ν₂ mode. The slowness of this process indicates the likelihood that the ν₁ and/or ν₃ modes (rather than the ν₂ mode) play a predominant role in the laser enhanced reaction between O₃² and NO at 298 K.