Photodissociation dynamics of tert-butylnitrite following excitation to the S1 and S2 states. A study by velocity-map ion-imaging and 3D-REMPI spectroscopy

Abstract

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 .

Tuning photochemistry: substituent effects on πσ* state mediated bond fission in thioanisoles

“…23. These fits have been observed previously [23][24][25] to be in very good agreement with the results of a direct Abel inversion by the matrix method 26 given sufficient signal to noise ratio.…”

Inverting ion images without Abel inversion: maximum entropy reconstruction of velocity maps

“…We observed previously [29][30][31][32] that fits with this function are in good agreement with the results of a direct Abel inversion by the matrix method 9 when the signal to noise ratio is high. Hence this function should produce realistic mock data for a test of our method.…”

“…At the optimum of H a small change in X will be compensated by the same change in Y, hence @S @X J ¼ À @S @Y J (32) with the consequence that X J Y J = B J…”

Near ultraviolet photochemistry of 2-bromo- and 2-iodothiophene: Revealing photoinduced ring opening in the gas phase?

“…2(b) shows, this can be further improved by increasing the aperture of the E electrode which, in the new design, comprises an entrance face defined by a sphere of radius 29.1 mm (centred on the TOF axis) and entrance and exit apertures of, respectively, 56 mm (as before) and 44 mm. As noted previously, 37 use of a spherical entrance surface also ensures that the desired linear relationship between image radius (r) and ion velocity (v) extends all the way to r = 0.…”