Determination of nascent hydrogen rovibrational distributions by laser-induced fluorescence: Applications to formaldehyde photodissociation and hydrogen exchange reactions

Individual quantum states of H{dollar}sb2{dollar} have been detected using the technique of Laser Induced Fluorescence. This technique is extremely sensitive, with a detection sensitivity of 6 {dollar}times{dollar} 10{dollar}sp8{dollar} states cm{dollar}sp{lcub}-3{rcub}{dollar} in a probed volume of 0.029 cm{dollar}sp3{dollar} with a signal-to-noise ratio of 3. This technique is also accurate. A spectrum of room temperature H{dollar}sb2{dollar} had a rotational temperature of 305.8 {dollar}pm{dollar} 3.0 K. LIF detection of H{dollar}sb2{dollar} was used to study the H{dollar}sb2{dollar}CO photodissociation reaction under essentially collision-free conditions. Following excitation of the {dollar}sp{lcub}r{rcub}Rsb3{dollar} bandhead of the 2{dollar}sbsp{lcub}0{rcub}{lcub}1{rcub}{dollar}4{dollar}sbsp{lcub}0{rcub}{lcub}3{rcub}{dollar} transition at 30388.5 cm{dollar}sp{lcub}-1{rcub}{dollar}, the principal product was ortho-H{dollar}sb2{dollar}. States as high as v = 4, J = 5 were seen. Within the v = 1 channel, the rotational distribution peaked at J = 3. Following excitation of the {dollar}sp{lcub}r{rcub}Rsb4{dollar} bandhead of the 2{dollar}sbsp{lcub}0{rcub}{lcub}1{rcub}{dollar}4{dollar}sbsp{lcub}0{rcub}{lcub}3{rcub}{dollar} transition at 30396.7 cm{dollar}sp{lcub}-1{rcub}{dollar}, the principal product was para-H{dollar}sb2{dollar}. This is the first report of the para-H{dollar}sb2{dollar} distribution following H{dollar}sb2{dollar}CO dissociation. The rotational distribution is similar to that for ortho-H{dollar}sb2{dollar} and peaks at J = 4 within v = 1 channel. This result contradicts a recent theoretical calculation that predicted that the para-H{dollar}sb2{dollar} distribution would be bimodal with a minimum at J = 6. Quantitative analysis of higher vibrational levels was hampered by a lack of spectroscopic knowledge (both transition energies and intensities) when the {dollar}tilde B{dollar} and {dollar}tilde C{dollar} states perturb each other. Attempts to use LIF to study the hydrogen exchange reaction have been unsuccessful.