Manifestation of intramolecular vibrational energy redistribution on electronic relaxation in large molecules

Some universal characteristics are discussed of the decay lifetimes and fluorescence quantum yields from the S₁ manifold of large molecules, which originate from the coupling between intrastate vibrational energy redistribution and interstate electronic relaxation. The time-resolved total fluorescence decay from the S₁ state of jet-cooled 9-cyanoanthracene exhibits non-exponential decay in the energy range E_v= 1200–1740 cm⁻¹ above the S₁ origin, which does not originate from dephasing but rather manifests the effects of intrastate intermediate level structure for vibrational energy redistribution on intersystem crossing.     

Study of single vibrational level relaxation in CS2(R3B2) by stimulated emission pumping

The relaxation of single vibrational levels of CS₂(R³B₂) has been studied by the stimulated emission pumping technique for the first time. The relaxation rate of the 0,6,0 and 0,8,0 levels is found to be 0.89 × 10⁸ s⁻¹ Torr⁻¹ and 1.2 × 10⁸ s⁻¹ Torr⁻¹ respectively. These values are about an order of magnitude higher than the R³B₂ state quenching rate. This is direct experimental evidence that the vibrational relaxation process within this triplet takes place essentially prior to quenching.     

Raman study of molecular dynamics of inorganic fluoroxidizers in nonaqueous solutions—Part 1. Xenon difluoride in acetonitrile

The Raman spectra of the totally symmetric ν₁(Σ_g⁺) mode of XeF₂ molecules have been measured in CH₃CN solutions at various temperatures and concentrations. The vibrational and rotational correlation functions as well as the characteristics times have been calculated. It was concluded that the vibrational band width in these solutions is to be attributed to the vibrational dephasing, whereas the contribution from the rotational relaxation has been found to be of less importance.     

Raman study of molecular dynamics of inorganic fluoroxidizers in nonaqueous solvents—Part 2. Xenon difluoride in hydrogen fluoride and bromine pentafluoride

The Raman spectra of the totally symmetric ν₁(Σ⁺_g) mode of xenon dilluoride molecules have been measured in HF and BrF₅ solutions at various concentrations. It is shown that the XeF₂ spectrum in HF has a complicated structure with three visible peaks. The complicated contour was resolved on elementary components which were identified. The vibrational and rotational correlation functions as well as the characteristics times have been calculated. It was concluded that the ν₁ vibrational band width of XeF₂ molecules in HF and BrF₅ solutions is to be attributed to the vibrational dephasing, whereas the contribution from the rotational relaxation has been found to be of less importance.     

Vibrational dephasing and energy relaxation of admolecules by phonons

An investigation is made of the vibrational dephasing of a diatomic molecule adsorbed on a surface. Explicit analytic forms for the rate of dephasing by phonons are derived. For comparison, an expression for energy relaxation is given which is appropriate for OH on SiO₂. It is found that the dephasing rate is considerably faster for this system than the energy relaxation rate. These conclusions are compared with the results of a recent experiment.     

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.     

Electronic and Vibrational Coherence Dynamics in a Cyanine Dye Studied Using a Few‐Cycle Pulsed Laser

We report the relaxation times of electronic and vibrational coherence in the cyanine dye 1,1′,3,3,3′,3′‐hexamethyl‐4,4′,5,5′‐dibenzo‐2,2′‐indotricarbocyanine, measured using a 7.1 fs pulsed laser. The vibrational phase relaxation times are found to be between 380 and 680 fs in the ground and lowest excited singlet states. The vibrational dephasing times of the 294, 446, and 736 cm^?1 modes are relatively long among the six modes associated with excited‐state wave packets. The slower relaxations are explained in terms of a coupled triplet of vibrational modes, which preserves coherence by forming a tightly bound group to satisfy the condition of circa conservation of vibrational energy. Using data from the negative‐time range (i.e., when the probe pulse precedes the pump pulse), the electronic phase relaxation time is found to be 31±1 fs. The dynamic vibrational mode in the excited state (1171 cm^?1), detected in the positive‐time range, is also studied from the negative‐time traces under the same experimental conditions.     

Microviscosity dependence of the Raman band shape of methyl-isobutyl ketone

The Raman band shape analysis of the CO stretching mode of vibration of methyl-isobutyl ketone in solution phase reveals that macroscopic consideration of hydrodynamic force is not sufficient to correlate the vibrational relaxation rate (τ_v⁻¹) with parameter ƒ(ϱ, η, n), involving dynamic viscosity (η). The band shape analysis was therefore attempted taking the microscopic parameter, microviscosity (η_m) into account through a modified parameter ƒ_m. The correlation of τ_v⁻¹ with ƒ_m is reported.     

Investigation of the phonon band gap effect on Raman-active optical phonons in SrMoO4 crystal

The phonon dispersions of SrMoO₄ crystal are calculated using the lattice dynamical calculations approach. Spontaneous Raman spectra in the SrMoO₄ were measured in the temperature range from 10 K to 295 K, and the temperature dependence of the linewidth of the B_g (95 cm⁻¹) and A_g (888 cm⁻¹) Raman modes was analyzed using the lattice dynamical perturbative approach. We found that different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the phonon spectrum resulting in different anharmonic interactions. The calculated temperature dependence of the linewidth of A_g (888 cm⁻¹) mode was well accounted for the experimental one by including both down-conversion by the cubic term and the dephasing by quartic term. The dephasing processes are increased only at high temperatures and the effect of dephasing is related to the size of a large phonon band gap.     

Vibrational relaxation of liquid dichloromethane at high pressure

The Raman spectra of two symmetric bands ν₁ and ν₃ of CH₂Cl₂ have been measured as a function of pressure to 300 MPa (3 kbar) and over the temperature range 303–363 K. For all bands the isotropic width increases with inreasing pressure and temperature. The experimental vibrational relaxation times are compared with the predictions of different combination of mass factors using the Fischer—Laubereau vibrational dephasing model.     

Vibrational Dephasing in Ionic Liquids as a Signature of Hydrogen Bonding

Understanding both structure and dynamics is crucial for producing tailor‐made ionic liquids (ILs). We studied the vibrational and structural dynamics of medium versus weakly hydrogen‐bonded C?H groups of the imidazolium ring in ILs of the type [1‐alkyl‐3‐methylimidazolium][bis(trifluoromethanesulfonyl)imide] ([C_nmim][NTf₂]), with n=1, 2, and 8, by time‐resolved coherent anti‐Stokes Raman scattering (CARS) and quantum‐classical hybrid (QCH) simulations. From the time series of the CARS spectra, dephasing times were extracted by modeling the full nonlinear response. From the QCH calculations, pure dephasing times were obtained by analyzing the distribution of transition frequencies. Experiments and calculations reveal larger dephasing rates for the vibrational stretching modes of C(2)?H compared with the more weakly hydrogen‐bonded C(4,5)?H. This finding can be understood in terms of different H‐bonding motifs and the fast interconversion between them. Differences in population relaxation rates are attributed to Fermi resonance interactions.     

Vibrational relaxation of NH2|X2B1(O,v2, O)| radicals

Energy transfer processes in NH₂ radicals have been studied using the sensitive laser-induced fluorescence (LIF) technique. The NH₂ radicals were generated by infrared multiple-photon dissociation (IR MPD) of monomethylamine (CH₃NH₂), and the state-selected NH₂(v″₂ = 1) decay was observed by the LIF detection of [NH²]. The vibrational relaxation processes studied are NH₂(v″₂ = 1) + M → NH₂(v″₂ = O)+M, with M  He, Ne, Ar, Kr, H₂, D₂, CO, O₂, and total decay rate of NH₂(v″₂ = 1) in the presence of excess of CH₃NH₂. Rate constants of (3.41±0.03)×10⁻¹³, (1.75±0.09)×10⁻¹³, (3.03±0.08)× 10⁻¹³, (3.58±0.06)×10⁻¹³, (13.4±0.5)×10⁻¹³, (4.70±0.19)×10⁻¹³, (4.3±0.3)×10⁻¹³, (5.9±-0.4)×10⁻¹³, (9.2±0.5)×10⁻¹³), and 8.4×10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were determined for the vibrational deactivation of NH₂(v″₂ = 1) by He, Ne, Ar, Kr, H², D₂, N₂, CO, O₂, and CH₃NH₂, respectively. The effect of the different collision partners on the relaxation rate is discussed. The results can be qualitatively well understood in terms of strong vibration—rotation coupling, due to the small moment of inertia of the NH₂ radicals.     

Rotational effects on intramolecular radiationless transitions in a large molecule

We report the observation of a pronounced rotational-state dependence for the fluorescence quantum yield and fluorescence decay from the S₁(0,0+1160 cm⁻¹) state of jet-cooled 9-cyanoanthracene. We attribute these results to the effects of Coriolis rotational-vibrational interaction on intramolecular vibrational energy redistribution, which influence electronic relaxation.     

The emission spectrum of the 1B1(π*-n+) state of 1,2-cyclobutanedione excited at 488.0 nm

The spectrum of the emission from the ¹B₁(π^*-n₊) state of 1,2-cyclobutanedione excited at 488.0 nm has been measured. Wavelengths and vibrational assignments are reported for 24 bands between 490 and 550 nm, 12 of which can be identified with hot bands in the absorption spectrum. Prominent bands in the emission spectrum are associated with excitation of V^''₈, the symmetric in-plane carbonyl bend (281 cm⁻¹); v^''₁₂, the asymmetric carbonyl wag (488 cm⁻¹); and v^''₇, a symmetric ring distortion (522 cm⁻¹). Sequences in v₁₃, the ring-twisting vibration, are also prominent; the initial excitation lies in the 13³₃ absorption band, while the emission shows intensity maxima for v^'₁₃ = 0 and 2, and a bimodal vibrational relaxation is suggested.     

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.     

Two-photon sequential absorption spectroscopy of the E(0+) state of iodine monofluoride

The energy levels of a new state of iodine monofluoride lying 5 eV above the ground state have been measured using the two-photon sequential absorption technique. Eleven vibrational levels of the state were measured. A vibrational and rotational analysis gives the spectroscopic constants as T_e = 41291.96 cm⁻¹, ω_e = 248.76 cm⁻¹, ω_eχ_e = 0.4951 cm⁻¹ and B_e = 0.12920 cm⁻¹. The state has 0⁺ character, and dissociates to I⁺ + F⁻ in the diabatic approximation.     

The vibrational relaxation of I2 by H2 in a supersonic jet

The vibrational relaxation of I₂ by H₂ has been studied in a supersonic free jet. It was observed that the addition of 5% H₂ to the helium carrier gas greatly reduces the concentration of X ¹Σ⁺_g(ν″ = 1) I₂ in the jet as compared to the concentration in a pure helium carrier. From this observation we have determined that the average vibrational relaxation cross sections of H₂ is 7.1 times as large as that of helium. Since the average vibrational relaxation cross section of deuterium is at least as large as that of hydrogen, the mechanism responsible for this phenomenon appears not to be dominated by mass effects.     

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.     

Hydrogen bonded complexes and the HF vibrational energy distributions from the reaction of F atoms with NH2 and NH3

The gas phase reactions of fluorine atoms with amino radicals and ammonia molecules: F(²P)+NH₂(²B₁) → HF(¹Σ⁺)+NH(³Σ⁻) and F(²P)+NH₃(¹A₁) → HF(¹Σ⁺)+NH₂(²B₁) produce hydrogen fluoride with very different primary vibrational energy distributions as determined by low-pressure chemiluminescence studies. The reaction with NH₂ yields HF with an inverted primary vibrational energy distribution, P(v′=1:2:3:4)=0.23:0.68:0.08:0.01. The HF from the reaction with ammonia is cold (non-inverted), P(v′=1:2)=0.60:0.40. Recent experimental work on these reactions is critically assessed and some discrepancies between low-pressure chemiluminescence results and fast-flowing afterglow studies are resolved. The results of high-level ab initio calculations (up to 6–311G^** CISD) on reactants, products, and the hydrogen bonded complexes FH … NH and FH … NH₂ in the exit channels are reported. The most reliable of the computations predict that FH … NH₂ is significantly more bound than FH … NH (8.1 versus 4.1 kcal mol⁻¹ in comparison with products at the 6-311G^** MP2 level).Also, the calculated vibrational frequencies for the two hydrogen bonded complexes indicate that the FH stretch and NH₂ asymmetric stretch are much closer in frequency in FH … NH₂ than are the FH and NH stretches in FH … NH. The strong interaction and the close match of vibrational frequencies in the FH … NH₂ case both will lead to fast internal vibrational relaxation (IVR) of the reaction exoergicity from the FHN bonds, where it is released, to the NH₂ fragment in the F/NH₃ reaction. Thus, the HF produced in this reaction is expected to have less vibrational excitation than that created in the F/NH₂ reaction, for which these IVR mechanisms are not as important, and simple direct abstraction dynamics are expected.