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The photodissociation of the double bond in HN=NH yielding electronically excited NH(A 3Π) and ground state NH(X 3Σ−) radicals has been studied in the vacuum-ultraviolet above 105 nm. Fragment excitation spectra were taken using tunable synchrotron radiation as the photolysis light source. The excited radicals were detected by their triplet emission to the ground state. A very crude estimate results in 10%, 20%, and 70% of the excess energy to be channeled into fragment vibration, rotation, and translation, respectively, at the Kr resonance line at 123.6 nm. This energy distribution supports a repulsive process with almost equal rotation in the two NH fragments and vibration caused by lengthening all bonds during the N=N bond breaking. An upper limit for the energy necessary to break the double bond is measured to be 510.7±1.2 kJ mol−1. This value yields ΔfH00(N2H2)≥204.1±2.2 kJ mol−1.

The photodissociation of NH2→NH(A 3Π)+H was investigated by photolyzing NH2 in a flow system with tunable synchrotron radiation from 200 to 105 nm and other vuv light sources. The NH photofragments were analyzed by their triplet emission at 336 nm. Additionally, ab initio configuration interaction calculations were performed for the electronic states of NH2 involved in the photodissociation process. Vertical excitation energies, bending potentials for the excited states, Franck–Condon factors, and transition moments were calculated in order to interpret the experimental observations. The following picture evolves for the dynamics of the NH2 photodissociation: At about 7.8 eV, NH2 is excited to the 2 2A1(A′) state, which possesses the same bending angle as the X̃ 2B1 ground state. The upper state correlates with the fragments NH(A 3Π)+H. Since the bending angle is not changed, the NH(A) radicals are formed with little rotational excitation. However, the symmetric stretch becomes excited at the beginning of the dissociation leaving the NH(A) fragment with vibrational excitation. Because of symmetry conservation, the formation of the Π(A′) component of NH(A) is preferred. In the region of ∼9 eV, transitions to the 1 2A2 and/or 3 2B1 states (both have A″ symmetry in Cs) occur. The bending potentials of both states have minima for linear configurations. Therefore, the structure of the excitation spectrum is determined by a progression in the bending motion and a preferred population of high rotational NH(A) levels is observed. Vibrational excitation is small suggesting that the unbroken NH bond stays unchanged during the dissociation process. According to symmetry conservation, the Π(A″) component of NH(A) is preferably formed.

The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2 ã 3B1–X̃ 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2 Ã 1B1–X̃ 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the D̃ 2A1 state of SiF4+ predominantly in the range 280–350 nm for photon energies greater than 21.5 eV. These assignments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection at a specific (dispersed) wavelength. Using the single-bunch mode of the synchrotron, lifetimes of all the emitting states have been measured. The lifetimes of the unassigned emitting state in SiF3, the Ã 1B1 state of SiF2, and the D̃ 2A1 state of SiF4+ are 3.9±0.7, 11.2±1.5, and 9.16±0.02 ns, respectively. This is the first measurement of the lifetimes of these excited states in SiF3 and SiF2. The decay from the ã 3B1 state of SiF2 has a fast component of 2.6±0.4 ns. We conclude that the lifetime of the ã 3B1 state of SiF2 is either as low as 2.6 ns or too high (τ>∼200 ns) to measure with the timing profile of the single-bunch mode of BESSY 1. If the latter interpretation is correct, as seems likely for a spin-forbidden phosphorescence to the A11 ground state, the 2.6 ns component could be the lifetime of intersystem crossing from higher vibrational levels of the ã 3B1 state of SiF2 into its ground state.