Photodissociation of NO2 at 355 and 351 nm Investigated by Photofragment Translational Spectroscopy

Abstract

The photodissociation of jet-cooled NO:! at 355 and 351 nm has been investigated by polarized high-resolution photofragment translational spectroscopy. The translational energy distributions P(&) of the nascent photofragment pairs NO + 0 were derived from the measured time-of-flight (TOF) distributions. By comparison of P(ET) with the available energy, the population of vibrationally excited NO was determined to be P(v=O) = 62 f 3% and P ( v = l ) = 38 F 3% at 355 nm and P(v=O) = 57 f 3% and P ( v = l ) = 43 F 3% at 351 nm. These findings are consistent with the trend predicted by statistical models of the dissociation process. Nonstatistical decay dynamics are, however, indicated for the rotational degrees of freedom as manifested by bimodal (or multimodal) rotational distributions of NO(v=O, 1). The recoil anisotropy parameter p as a function of fragment translational energy was obtained from the polarized TOF spectra and was found to depend on the photolysis wavelength and on the vibrational state of the NO product: p(v=O) = 1.42 and p ( v = l ) = 1.25 at 355 nm, whereas p(v=O) = 1.77 and p ( v = l ) = 1.48 at 351 nm. The NO rotational alignment A f ) measured by the laser-induced fluorescence method is at high J values close to the theoretical (perpendicular-type) limit of -0.4 at both photolysis wavelengths. This result, in conjunction with the values of p, implies that the photon absorption occurs via the : !B: ! -electronic transition and that the photodissociation takes place essentially without anisotropy loss and hence on a subpicosecond time scale.The observed dependence of p on the product vibrational state suggests two different decay pathways with quantum mechanical effects playing an important role.

Photodissociation dynamics of à state ammonia molecules. I. State dependent μ-v correlations in the NH2(ND2) products

“…396 nm͒ photolysis of NO 2 ͑Ref. 48͒ and for the vϭ0 and vϭ1 NO(X) products resulting from NO 2 photolysis at both 351 and 355 nm, 49 although our understanding of the number and nature of the excited electronic states of NO 2 involved in these dissociations precludes particularly detailed interpretation of these results.…”

Slow Molecules Produced by Photodissociation

“…If the laser is linearly polarized, the photofragments will have an angular distribution in terms of the angle between the laser polarization and the recoil velocity [14,22],…”

“…This discrepancy is probably due to the contributions from the ionization of NO molecules in a number of rotational states. Since about 40% of the NO fragments are in the first vibrationally excited state [22], assuming that about 50 rotational states are equally populated, 0.03% of the NO molecules produced by photodissociation would be decelerated propagating along the z axis might be manipulated with a molecule prism [16]. When the molecular beam velocity is reduced from 565 m/s to 35 m/s, the velocity change due to the molecule prism of a laser pulse with a 5 μm waist radius and 10 ns pulse width will be enhanced by 80%.…”

The 248 nm photolysis of NO2/N2O4: Time-resolved Fourier transform infrared emission from NO and NO2, and quenching of NO (v=5–8)

“…350 nm, 25 but cannot predict vibrational populations which show population inversions and thus cannot apply here. Absorption at 248 nm is close to a minimum value in the spectrum of NO 2 , 12 to the short wavelength side of the onset of the second excited 2 2 B 2 state, 6 and in a region which has been assigned to the high J tail of the 2 2 B 2 (0,1,0) -X 2 A 1 (0,0,0) band.…”