State-to-state vibrational relaxation in 1B2u benzene-d6 induced by low energy collisions with He, H2, N2, and Ar

Abstract

Time-resolved dispersed fluorescence spectroscopy has been used to study very low energy collision-induced vibrational relaxation in 1B2u benzene-d6 expanded in a supersonic free jet. Several expansion gases have been used to enable rate coefficients for vibrational relaxation to be obtained as a function of collision partner in the very low energy collision regime. Benzene-d6 is found to undergo vibrational deactivation with each of the chosen collision partners He, H2, N2, and Ar. This study focuses on measuring state-to-state relaxation rate coefficients and branching ratios for vibrational relaxation from the S1 vibrational level 61 (εvib =498 cm−1). Relaxation occurs to all the known S1 vibrational levels lying lower in energy than 61, namely 101 (εvib =454 cm−1), 162 (εvib =414 cm−1), 111 (εvib =382 cm−1), 41 (εvib =306 cm−1), 161 (εvib =207 cm−1), and 00 (εvib =0 cm−1). Two other unidentified channels also participate. Nitrogen and argon are found to be the most efficient collision partners in inducing relaxation (σobs/σhs ∼1.1, 1.7, respectively), whereas hydrogen and helium are found to be an order of magnitude less efficient (σobs/σhs ∼0.08, 0.02, respectively). The relaxation channels 61→101 and 61→111 are the dominant deactivation pathways for all the gases studied. The relative importance of relaxation pathways changes with collision partner. There appears to be a light or small collision partner effect, whereby the propensity for transfer through the channel 61→101 (Δεvib =−44 cm−1) seems to be redistributed among other channels, involving a greater energy or momentum gap, as the collision partner becomes lighter.

Vibrational Energy Transfer

“…The state-to-state VET competition in the very low energy collision domain of cold jets has been monitored for one initial S 1 level and several collision partners in benzene, , benzene- d 6 , , and naphthalene . It has also been observed from several levels in S 1 p -difluorobenzene, ,, again with several collision partners.…”

“…Molecules whose study at 300 K was precluded by thermal congestion in S 1 −S 0 absorption become accessible in cold jets. Naphthalene with 48 modes is a prime example of a complicated vibrational system opened for VET study by the cold jet approach. − Another is C 6 D 6 , a difficult case at 300 K.…”

Conformational Isomerization and Collisional Cooling Dynamics of Bis(2-hydroxyphenyl)methane

“…Studies of collision-induced vibrational energy transfer have a long history. − With the invention of tunable lasers, it became possible to directly prepare specific vibrational levels and follow vibrational relaxation. Low in the energy manifold, state-to-state propensities can be mapped out using pump−probe methods or by state-resolving the emission from the product levels. − For large molecules with significant amounts of internal energy, state-to-state studies are no longer possible, but various ingenious methods have been developed to determine the average energy lost per collision as a function of initial internal energy, up to internal energies of 100 kcal/mol or more (35 000 cm −1 ). ,,, From the viewpoint of vibrational energy transfer, 2HDPM is in a class of molecules which has not received much attention previously. As we have already indicated, 2HDPM has two spectroscopically distinguishable minima on a model 4D potential energy surface, involving the phenyl and OH torsions.…”

State-to-state vibrational energy transfer from 61 benzene induced by low-energy collisions with nitrogen: temperature-dependent propensities

“…Rainbird et al have presented the kinetic scheme appropriate for time-resolved measurements of state-to-state vibrational relaxation rates. 3 In the "single-collision" regime, which is appropriate for our data, the time-dependent population of a collisionally populated level is given by collisionally populated level, TV denotes the population, ' is the total removal rate constant for level i (it includes the radiative, nonradiative, and collision rate constants), and is the stateto-state rate constant for population transfer from i -*• g. Thus, we are interested in extracting the ratios of the £'s for each of the collisionally populated levels. Under our conditions of pressure, and hence total vibrational relaxation rate, the exponential termis small over the time duration of the experiment, and this expression becomes…”