The photodissociation of C&OE\IO, t-(CH3)3CON0, and the clusters [CH3ONO],, with n = 2 to -20, and [(CH3)3CONO],, with n = 2 to -15, was investigated in a supersonic jet using laser excitation to the Sl(nr*) state and 2 + 1 LIF fragment state probing of the N O product including sub-Doppler line profile measurements.We determined the vibrational, rotational, and translational energy distributions as well as the rotational alignment A,+*) and the anisotropy @ of N O emerging from monomer and clusters. The results of CH3ONO in the beam are similar to those found previously in the bulk phase and those of (CH3)3CONO parallel the findings of CH3ON0, implying a decay mechanism which involves direct dissociation and predissociation with the latter being dominant in both molecules. Photodissociation of the weakly bonded [CH3ONO], clusters produces NO fragments with low rotational energy and almost no translational energy = 0; / 3 = 0) but with a vibrational energy distribution equal to that of the monomer. For the stronger bonded cluster [(CH3)3-CONO], also the vibrational excitation of N O is significantly relaxed. On the basis of these results mechanistic features of the cluster photodissociation are proposed.

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.

Homogeneous and neutral clusters of tert-butyl nitrite [ (CH3)3CONO],, with (n) roughly 20, were produced by adiabatic jet expansion and photolyzed via the localized Sl(nn*) -SO transition to R -I-NO (21J) at A d * 360-400 nm. Using photofragment yield spectroscopy (PHOFRY), we detennined the relative partial absorption cross sections leading to NO (v" = 0, 1, 2) products from the monomer and the clusters which allowed us to selectively investigate from a monomer/cluster mixture the cluster formation and the cluster photodissociation. The latter was further characterized by the rotational state distributions of the four sublevels of NO (21J*1/2 and '&3/2) and the rotational alignment A ! ) .Essentially two types of NO fragments emerged from the photoexcited clusters: one having relaxed J state distributions and no alignment A!), the other hot (non-Boltzmann) distributions and a strong alignment as well as a preference for the A-state A". The relaxed NO fragments are proposed to arise from nitrite molecules "solvated" in the cluster and the hot NO fragments from nitrite molecules with a free chromophore located on the surface of the cluster. The vibrational state population of NO appears no to be significantly influenced by cluster formation.

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