ROTATIONAL RELAXATION IN THE $v_{4}=1$ VIBRATIONAL LEVEL OF $H_{2}CO\tilde{A}^{1}A_{2}$ BY TRANSIENT GAIN SPECTROSCOPY

Author Institution: Department of Chemistry, Massachusetts Institute of Technology; Department of Chemistry, Texas Tech University; Huygens Laboratorium, Rijksuniversiteit LeidenRotational relaxation in the $v_{4}=1$ out-of-plane bending mode of $\tilde{A}^{1}A_{2}$ formaldehyde is investigated by means of a novel, folded double-resonance technique. A pulsed dye laser is tuned to coincide with a specific rovibronic transition in the $\tilde{A}^{1}A_{2}\to \tilde{A}^{1}A_{1} 4^{1}_{0}$ band. The transient gain appearing on a copropagating cw probe laser, coincident with the $2^{0}_{1}3^{0}_{1}4^{1}_{4}$ transition, samples total depopulation rates as well as rotational state-to-state cross sections. Results verify that relaxation in the formaldehyde $\tilde{A}^{1}A_{2} v_{4}=1$ level occurs with a rate approximately ten times the gas kinetic collision rate. The sensitivity of transient gain spectroscopy, in conjunction with its superior spectral $(0.05 cm^{-1})$ and temporal (6 nsec) resolution, is shown to be especially useful in the $H_{2}CO \tilde{A}^{1}A_{2}$ system where the radiative rate is slow compared to the strongly rotation level dependent rates of nonradiative processes