The photoelectron spectrum shows that multiphoton ionization of amyl nitrite, C(5)H(11)ONO, using ultrafast laser pulses deposits up to 3.7 eV of energy into internal degrees of freedom. As a result, the molecules fragment to produce various daughter ions of masses 87, 71, 60, 57, 41, 30, 29, and 27. Absorption of an additional photon with 3 eV of energy by the ions yields transients with picosecond decay times, revealing the time scale of the decomposition dynamics of the initially prepared parent ion. Each mass peak has a distinct time constant, in the range of 1.2 to 7.9 ps, emphasizing the dependence of the fragmentation mechanism on the ion internal energy.

We have investigated the processes induced by femtosecond laser pulses in chloroamines, with a focus on the generation and observation of a highly reactive radical and on the involvement and general importance of excited-state ions in time-resolved mass spectrometry investigations of gaseous molecules. We have found that 280 nm femtosecond pulses lead to an ultrafast breakage of the N-Cl bond on the repulsive S1 surface, and that resulting radical is long-lived. When exposing the molecule to 420 nm photons a multiphoton ionization takes place to generate ions; these ions can then be excited with a 280 nm photon. The evidence is unambiguous since we observe a distinct temporal evolution of the ion current with no photoelectrons to match. We suggest that the involvement of excited-state ions is a general phenomenon in time-resolved photoionization studies.

We have performed time-resolved photoelectron spectroscopy and mass spectrometry experiments to address the dynamics that result when gaseous acetone molecules are excited with femtosecond pulses in the 253–288nm wavelength range. There are several previous examples of time-resolved mass spectrometry experiments and our results are in line with previously published data. However, the results of the photoelectron spectroscopy experiments allow us to show that the ultrafast dynamics related to the S1 state can be attributed entirely to photo-physical processes. In essence, the dynamics that is induced by a one-photon excitation is governed by the motion out of the Franck–Condon region on the S1 surface to the relaxed geometry in less than 30fs. The relaxed S1 species does not decay in 100ps and actual C−C bond breakage must take place on a longer time scale.

We have performed ab initio calculations to examine the potential energy along the normal modes of ground-state HCHO and along the reaction coordinates for loss of H2 and atomic hydrogen, respectively. This exploration showed that there are no specific features that will lead to reaction on the excited-state surfaces for excitations that are relevant to the troposphere and stratosphere. The calculations did however lead to the localization of a conical intersection point through which a specific loss of H2 could take place. However, the conical intersection lies at 5.4 eV relative to the ground state molecule at equilibrium and is thus inaccessible via single photon excitation at tropospheric and stratospheric wavelengths. In addition to the ab initio investigation we have carried out a femtosecond pump-probe experiment using a 266/400 nm excitation. The results show that the timescale for the internal conversion from the initially prepared high-lying Rydberg states is on the order of a picosecond. This process populates the n --> pi* first excited singlet state which then survives for a substantially longer time before it is depopulated to form hot ground state or triplet-excited molecules that can then decompose.