A new data acquisition mode has been implemented to a velocity-map ion-imaging setup, which records the velocity distributions of molecular photofragments with vibrational and rotational resolution. Compared to conventional velocity-map ion-imaging, the acquired data are of remarkable brilliance. This allows for unambiguous assignment of the fragment quantum states and the analysis of all rotational bands apparent in the electronic transition of the molecular fragment. The acquisition time is the same as required for recording a REMPI spectrum of the photofragments. The method is illustrated by the measurement of the rotational state distribution of NO created in the photolytical decomposition of NO(2) at 225 nm. Different rotational distributions were observed for each vibrational state and for each of the four energetically accessible electronic channels.
S-Nitrosothiols serve as carriers and donors of NO in several important biological signaling systems. In these compounds the S-NO bond is rather labile and NO can be released thermally or photochemically. This paper reports on the photolytical decomposition of tert-butylthionitrite (t-BuSNO) in the visible and near-UV spectral regions. Between 500 and 605 nm several vibronic levels of the S(1) (npi*) state were excited, including the electronic origin. At 360 nm t-BuSNO is excited near the maximum of the first UV band assigned to the S(2) (pipi*) state. The velocity distributions of several hundred rovibrational states of the NO fragments were recorded with the recently developed 3d-REMPI method. A global fit to these data yields populations of the rovibrational states in both spin-orbit components of the (2)Pi electronic state of NO as well as their velocity distributions and angular anisotropies beta. These data also carry the distribution functions for internal and kinetic energy of the counterfragment, the t-BuS radical. The range found for the anisotropy parameter confirms the npi* character of the visible absorption band (-1.0 < beta < -0.8), and the pipi* character of the UV band (beta = 1.2). Mode-specific dissociation has been observed for excitation into several vibronic bands of the S(0) → S(1) transition. Some produce NO exclusively in the ν = 0 vibrational ground state, whereas some others produce NO almost entirely in the ν = 1 vibrationally excited state. It is concluded that photodissociation is faster than relaxation of the NO stretch vibration of t-BuSNO in S(1) and that it proceeds on purely repulse potential energy surfaces in both electronic states.
Excitation of tert-butylnitrite into the first and second UV absorption bands leads to efficient dissociation into the fragment radicals NO and tert-butoxy in their electronic ground states 2 P and 2 E, respectively. Velocity distributions and angular anisotropies for the NO fragment in several hundred rotational and vibrational quantum states were obtained by velocity-map imaging and the recently developed 3D-REMPI method. Excitation into the well resolved vibronic progression bands (k = 0, 1, 2) of the NO stretch mode in the S 1 ' S 0 transition produces NO fragments mostly in the vibrational state with v = k, with smaller fractions in v = k À 1 and v = k À 2. It is concluded that dissociation occurs on the purely repulsive PES of S 1 without barrier. All velocity distributions from photolysis via the S 1 (np*) state are monomodal and show high negative anisotropy (b E À1). The rotational distributions peak near j = 30.5 irrespective of the vibronic state S 1 (k) excited and the vibrational state v of the NO fragment. On average 46% of the excess energy is converted to kinetic energy, 23% and 31% remain as internal energy in the NO fragment and the t-BuO radical, respectively. Photolysis via excitation into the S 2 ' S 0 transition at 227 nm yields NO fragments with about equal populations in v = 0 and v = 1. The rotational distributions have a single maximum near j = 59.5. The velocity distributions are monomodal with positive anisotropy b E 0.8. The average fractions of the excess energy distributed into translation, internal energy of NO, and internal energy of t-BuO are 39%, 23%, and 38%, respectively. In all cases B8500 cm À1 of energy remain in the internal degrees of freedom of the t-BuO fragment. This is mostly assigned to rotational energy. An ab initio calculation of the dynamic reaction path shows that not only the NO fragment but also the t-BuO fragment gain large angular momentum during dissociation on the purely repulsive potential energy surface of S 2 .
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