Vibrationally excited populations from IR-multiphoton absorption. I. Absorbed energy and reaction yield measurements

Abstract

The molecule 1,1,2-trifiuroethane (TFE) was used in experiments to determine the population distribution of excited molecule3 produced by infrared multiphoton absorption induced by high power TEA C0 2 1asers operating at 1079.85 cm-1 [9.6I"m R (22) line]. Optoacoustic measurements of absorbed laser power proVided a measure of the mean energy of the population distribution, wl11je very low pressure photolysis measurements of the collision-free decomposition yield gave information about the high,energy tan of the distribution. The experimental results were accurately simulated using a Master Equa.tion model that incorporated Quack's statisticaldynamical theory of infrared mUltiphoton absorption (cases B and C), RRKM unimolecuJar reactions (three channels), and collisional energy transfer. The computer simulations included known TFE mo1ec'lJlar properties and oIlly four Ildjustable pl~ra.meters, which were very highly constrained in order to fit the {lxperiInt"Iltal data.. From the simuJatio,lS, 'We conclude that the opticaJ coupling matrix elements are dnunaHcaHy reduced in ma.g:llitude for energies above the reaction thresholds. This effect is symptomatic of the vibrational anllannonicity due to the presence of the reaction channels, even in molecuk>s that have not yet reacted, resulting in vibrational frequency shifts ofthe absorption lines out of resonance with the laser line. This effect is expected to be present and ohservable in oth!!r highly vibrationally excited molecules.

Energy transfer of highly vibrationally excited molecules studied by crossed molecular beam/time-sliced velocity map ion imaging

“…T energy transfer is more than 1000 cm À1 , DE d 5 À1000 cm À1 . The inserts in Figures 5,7,9,12 and 15 show the high energy tails of V ! T energy transfer.…”

“…Infrared fluorescence was used by Barker and co-workers to measure the energy transfer of hot molecules [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. In these experiments, the decay of hot molecule's internal energy due to collisional energy transfer to the bath molecules was monitored by infrared fluorescence emitted from hot molecules as a function of time.…”

A direct comparison of vibrational deactivation of hexafluorobenzene excited by infrared multiple photon absorption and internal conversion

“…Various workers have either calculated this distribution [3][4][5][6] or it has been inferred using experimental techniques such as Raman spectroscopy. 31,32 Problems arise as the distribution can be bimodal, resulting from a near thermal population of those molecules that did not absorb, and a higher energy distribution of molecules that absorbed many photons.…”

“…This arises because there is uncertainty in the initial energy distribution associated with the IRMPA process. [3][4][5][6] Nevertheless, it has been shown theoretically that under appropriate conditions the results extracted from the data depend solely on the average excitation energy, i.e., they are independent of the initial distribution. 10,11 The average internal energy the excited molecule initially reaches after IRMPA is controlled by varying the fluence of the excitation laser.…”

“…[2][3][4][5][6][7][8][9] In contrast with studies utilizing UV pumping followed by IC, IRMPA remains a poor cousin. This arises because there is uncertainty in the initial energy distribution associated with the IRMPA process.…”