Vibrational and Rotational Excitation of CO2 in the Collisional Quenching of H2 (v=1)

Energy transfer in H-2 (1,1) + CO2 collisions was investigated using high resolution transient laser spectroscopy. Rotational state selective excitation of v = 1 for rotational level J = 1 was achieved by stimulated Raman pumping. Energy gain into CO2 resulting from collisions with H-2 (1, 1) was probed using transient absorption techniques, Distributions of nascent CO2 rotational populations in both the ground (00(0)0) state and the vibrationally excited (00(0)1) state were determined from overtone absorption measurements. Translational energy distributions of the recoiling CO2 in individual rovibrational states were determined through measurement of Doppler-broadened transient line shapes. A kinetic model was developed to describe rates for appearance of CO2 states resulting from collisions with H-2 (1,1). From scanned CARS(coherent anti-stokes raman scattering) the spectral peaks population ratio n(0)/n(1) was obtained, where n(0) and n(1) represent the number densities of H-2 at the levels (0,1) and (1,1), respectively. Using rotational Boltzmann distribution of H-2 (v=0) at 300 K, n1 was yielded. Values for rate coefficients were obtained using data for CO2 (00(0)0) J=48 to 76 and CO2(00 1) J=5 to 33. The rate coefficients derived from appearance of the (00(0)0) state have values of k(tr) = (3.9 +/- 0.8) X 10(-11) cm(3) . molecule(-1) . s(-1) for J = 48 and k(tr) = (1.4-0.3) X 10(-10) cm(3) . molecule(-1) . s(-1) for J = 76, with a monotonic increase for the higher J states. For the (00(0)1) state, values of k(tr) remain fairly constant at k(tr) = (4.3 +/- 0.9) X 10(-12) cms . molecule . s(-1). Rotational populations for the nascent CO2 states were measured at 0.5 mu s following excitation of H-2. The transient population for each state was fit using a Boltzmann rotational distribution. The CO2 (00(0)0) J = 48 similar to 76 rotational states were populated substantially relative to the initial 300 K CO2 distributions, and the distribution is described by Trot. The excited (00(0)1) state has T-rot = 310 K. The center-of-mass translational temperatures for the (00(0)0) state are all much greater than 300 K, with T-rel =1 532 K for J = 76. In contrast, transient line profiles for the J=5 similar to 33 levels of excited (00(0)1) state do not show any broadening above the initial 300 K distributions, indicating that excitation to the (00(0)1) state is not accompanied by translational energy change.