Rovibrational Energy Transfer in the 4ν_CH Manifold of Acetylene, Viewed by IR−UV Double Resonance Spectroscopy. 1. Foundation Studies at Low J

Abstract

Time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy is used to prepare acetylene molecules (C 2 H 2 ) in specific rovibrational states of the 12 700 cm -1 4ν CH manifold of the electronic groundstate ˜, monitoring their direct excitation and collision-induced state-to-state energy transfer, by probing at ∼299 or ∼296 nm with laser-induced fluorescence via the Α electronic state. The 4ν CH manifold derives much of its IR brightness from the (ν 1 + 3ν 3 ) combination band, such that many of the rotational levels J monitored by IR-UV DR are derived from the (1 0 3 0 0) 0 vibrational state. The 4ν CH manifold of C 2 H 2 is congested and affected by anharmonic, l-resonance, and Coriolis couplings that cause other IR-dark, UVbright rovibrational levels to attain appreciable IR-UV DR intensity and to add to the complexity of intramolecular dynamics in that manifold. Consequently, collision-induced rovibrational satellites observed by IR-UV DR comprise not only regular even-∆J features but also supposedly forbidden odd-∆J features, of which the energy-transfer channel from J ) 12 to J ) 1 is particularly efficient. This paper focuses on low-J rovibrational levels of the 4ν CH manifold, particularly those with J ) 0 and J ) 1 in view of their anomalously large Stark effects that are likely to make them susceptible to collision-induced rovibrational mixing. Three complementary forms of IR-UV DR experiment are reported: IR-scanned, UV-scanned, and kinetic. These indicate that strong IR-UV DR signals observed by probing the (1 0 3 0 0) 0 J ) 0 rovibrational level are complicated by underlying IR-dark, UV-bright states, making J ) 0 unsuitable for systematic IR-UV DR studies. The (1 0 3 0 0) 0 J ) 1 rovibrational level is more amenable to unambiguous characterization and yields insight concerning even-and odd-∆J collision-induced rovibrational energy transfer and associated mechanisms.

Rovibrational Energy Transfer in the 4νCHManifold of Acetylene Viewed by IR−UV Double Resonance Spectroscopy. 2. Perturbed States withJ= 17 and 18

“…These spectra are relevant to results presented later in this paper. As before, 28 reduced term value plots 32,74,82 have been used to label P, Q, and R branches of the rotational structure and identify K′ in the upper vibronic state (ν′ 3 + ν′ 5 ), which combines the a g CC stretching and b u CH stretching modes. The primary subbands in Figure 3 are K 1 0 (i.e., K′ ) 1 r K ) l ) 0), but even-K′ features with K′ ) 0 and K′ ) 2 also occur.…”

“…Apart from these IR-UV DR investigations of the 12 700 cm -1 4ν CH manifold of C 2 H 2 , [28][29][30]32,33 we have also studied IR-UV DR spectroscopy and collision-induced energy-transfer dynamics in its 11 600 cm -1 (ν CC + 3ν CH ) manifold. [34][35][36][37] This complements other LIF-detected IR-UV DR studies of C 2 H 2 and its isotopomers in overtone/combination levels with high vibrational energy (G v > 6500 cm -1 ) [38][39][40][41][42][43] and fundamental and lower-energy overtone or combination levels (G v e 6500 cm -1 ).…”

“…Such odd-∆J energy transfer is understood to be possible in the presence of some forms of intramolecular and/or collision-induced rovibrational perturbations that are prevalent in the high-energy vibrational manifold of C 2 H 2 . [28][29][30][32][33][34][35][36][37] The IR-UV DR spectrum in trace a is recorded under effectively collision-free conditions with a very small collision number z. (As before, [28][29][30][34][35][36][37]47 values of z are referred arbitrarily to Lennard-Jones collisional rate constants k LJ that are consistent with those adopted by Crim and co-workers; 38,39 for C 2 H 2 /C 2 H 2 self-collisions at 300 K, k LJ ) 16.4 µs -1 Torr -1 ) 5.10 × 10 -10 cm 3 molecule -1 s -1 .)…”

“…[28][29][30][32][33][34][35][36][37] The IR-UV DR spectrum in trace a is recorded under effectively collision-free conditions with a very small collision number z. (As before, [28][29][30][34][35][36][37]47 values of z are referred arbitrarily to Lennard-Jones collisional rate constants k LJ that are consistent with those adopted by Crim and co-workers; 38,39 for C 2 H 2 /C 2 H 2 self-collisions at 300 K, k LJ ) 16.4 µs -1 Torr -1 ) 5.10 × 10 -10 cm 3 molecule -1 s -1 .) The simple spectrum in trace a comprises the (ν 1 + 3ν 3 )-band R(11) peak; it is accompanied by a single P(13) peak beyond the low-frequency limit of Figure 1, thereby confirming that the UV probe pulse is indeed monitoring the (1 0 3 0 0) 0 J ) 12 rovibrational level.…”

Control of frequency chirp in nanosecond-pulsed laser spectroscopy 2 A long-pulse optical parametric oscillator for narrow optical bandwidth

“…Continuously tunable, narrowband coherent light with ns pulse duration is also required for various spectroscopic applications that entail a timeresolved molecular excitation sequence, such as our infrared-ultraviolet double-resonance studies of J-resolved, collision-induced rovibrational energy transfer in acetylene (C 2 H 2 ). [6][7][8][9][10] In a recent publication 11 we reported a major advance in the techniques that are required for control of the optical bandwidth and frequency chirp characteristics of output from a single-longitudinal-mode (SLM), continuously tunable OPO system based on the quasi-phasematched NLO medium, periodically poled KTiOPO 4 (PPKTP). This PPKTP OPO was injection seeded by a cw tunable diode laser (TDL) at a signal wavelength s of ϳ842 nm and pumped at 532 nm by a SLM Nd:YAG laser with a pulse duration of ϳ8 ns.…”