In the photoelectron spectrum of N 2 the apparent ionization energy to form the B 2 ⌺ u + state increases linearly with the photon energy. Rotationally resolved measurements of the fluorescent decay of this state show a linear increase of rotational heating with increasing photon energy. These results are in quantitative agreement with the prediction of the theory of recoil-induced rotational excitation, indicating that the rotational heating that has been observed previously arises primarily from such recoil-induced excitation. Together with other results that have been reported they show that recoil-induced internal excitation is significant in many situations, including near threshold.When a photoelectron is ejected from an atom, molecule, or solid the remaining ion has a recoil momentum that is equal and opposite to that of the electron. Although the first discussions of this effect ͓1͔ were limited to translational recoil, it was recognized by Domcke and Cederbaum ͓2͔ that the recoil effect could lead to internal excitation of the ion. However, this prediction remained unverified until recent observations in core-electron photoelectron spectra of recoil excitation of vibrations in molecules ͓3,4͔ and phonons in solids ͓5͔. These experiments show that the recoil-induced internal excitation is quantitatively in accord with a model based on emission of the electron from a localized atom.Although a model based on emission from a localized atom may be appropriate for core ionization, it is not apparent that such a model is appropriate for valence ionization, where the electrons are delocalized. This question has been recently addressed by Takata et al. ͓6͔ who showed that at a photon energy of 8 keV there is a shift in the apparent position of the Fermi edge of aluminum that is consistent with the recoil being taken up by a single atom.The investigations mentioned above have been concerned with vibrational excitation. Here we consider recoil-induced rotational excitation during valence photoionization of N 2 . Thus we extend the previous investigations by considering a different type of internal excitation and by considering valence excitation in a distinctly different system ͑a small molecule rather than a solid͒. Specifically we investigate rotational excitation during photoionization to produce the B 2 ⌺ u + state of N 2 + . Using both photoelectron and fluorescence spectroscopy we show that there is recoil-induced rotational excitation in quantitative accord with a model based on emis-sion of the electron from a localized atom. Moreover, we note that this effect is observable even within 100 eV of threshold. Thus it becomes apparent that significant recoilinduced internal excitation is widespread in terms of both the physical system ͑molecule or solid͒ and the energy range.It has been previously noted that the distribution of rotational states produced during photoionization to form the B 2 ⌺ u + state of N 2 + depends on the photon energy ͓7,8͔. The distribution shifts to higher values of the rotational quan...

We report on the coherent control of the ultrafast ionization and fragmentation dynamics of the bromochloroalkanes C(2)H(4)BrCl and C(3)H(6)BrCl using shaped femtosecond laser pulses. In closed-loop control experiments on bromochloropropane (C(3)H(6)BrCl) the fragment ion yields of CH(2)Cl(+), CH(2)Br(+), and C(3)H(3)(+) are optimized with respect to that of the parent cation C(3)H(6)BrCl(+). The fragment ion yields are recorded in additional experiments in order to reveal the energetics of cation fragmentation, where laser-produced plasma radiation is used as a tunable pulsed nanosecond vacuum ultraviolet radiation source along with photoionization mass spectrometry. The time structure of the optimized femtosecond laser pulses leads to a depletion of the parent ion and an enhancement of the fragment ions, where a characteristic sequence of pulses is required. Specifically, an intense pump pulse is followed by a less intense probe pulse where the delay is 0.5 ps. Similarly optimized pulse shapes are obtained from closed-loop control experiments on bromochloroethane (C(2)H(4)BrCl), where the fragment ion yield of CH(2)Br(+) is optimized with respect to that of C(2)H(4)BrCl(+) as well as the fragment ion ratios C(2)H(2)(+)/CH(2)Br(+) and C(2)H(3)(+)/C(2)H(4)Cl(+). The assignment of the underlying control mechanism is derived from one-color 804 nm pump-probe experiments, where the yields of the parent cation and several fragments show broad dynamic resonances with a maximum at Δt = 0.5 ps. The experimental findings are rationalized in terms of dynamic ionic resonances leading to an enhanced dissociation of the parent cation and some primary fragment ions.

We report on fluorescence spectra of N(2)(+)(B (2)Sigma(u)(+)) --> N(2)(+)(X (2)Sigma(g)(+)) obtained from multiphoton ionization of molecular nitrogen by 804 nm femtosecond laser pulses. The analysis of the fluorescence spectra reveals that the vibrational levels v = 0 and v = 4 in the B (2)Sigma(u)(+)-state of N(2)(+) are primarily populated. The rotational state distribution of N(2)(+)(B (2)Sigma(u)(+), v = 0) is determined from the rotationally resolved fluorescence spectra. It is demonstrated that the linear chirp of the 804 nm femtosecond laser pulse has a strong influence on the rotational state distribution of the vibrational ground state of the molecular cation N(2)(+)(B (2)Sigma(u)(+), v = 0). Possible mechanisms leading to the experimental results are discussed. The particular population of the vibrational levels as well as the linear chirp dependence of the fluorescence signal gives evidence for the importance of a resonant intermediate state. The N(2) a (1)Pi-state is likely involved in a resonant multiphoton excitation process. This permits to selectively control the rotational population of the cation that is formed via chirped pulse multiphoton ionization.

Coherent excitation of a superposition of Rydberg states in neon by the 13th harmonic of an intense 804 nm pulse and the formation of a wave packet is reported. Pump-probe experiments are performed, where the 3d-manifold of the 2p6-->2p5 (2P3/2) 3d [1/2]1- and 2p6-->2p5 (2P3/2) 3d [3/2]1-transitions are excited by an extreme ultraviolet (XUV) radiation pulse, which is centered at 20.05 eV photon energy. The temporal evolution of the excited state population is probed by ionization with a time-delayed 804 nm pulse. Control of coherent transient excitation and wave packet dynamics in the XUV-regime is demonstrated, where the spectral phase of the 13th harmonic is used as a control parameter. Modulation of the phase is achieved by propagation of the XUV-pulse through neon of variable gas density. The experimental results indicate that phase-shaped high-order harmonics can be used to control fundamental coherent excitation processes in the XUV-regime.

Photoionization and autoionization of electronically excited atomic oxygen O(D1) are investigated in the energy range between 12 and 26eV using tunable laser-produced plasma radiation in combination with time-of-flight mass spectrometry. A broad, asymmetric, and intense feature is observed that is peaking at 20.53±0.05eV. It is assigned to the 2s22p4(D1)→2s12p5(P1) transition, which subsequently autoionizes by a Coster-Kronig transition, as predicted by the previous theoretical work [K. L. Bell et al., J. Phys. B 22, 3197 (1989)]. Specifically, the energy of the unperturbed transition occurs at 20.35±0.07eV. Its shape is described by a Fano profile revealing a q parameter of 4.25±0.8 and a width of γ=2.2±0.15eV. Absolute photoionization cross section σ is derived, yielding σ=22.5±2.3Mb at the maximum of the resonance. In addition, weak contributions to the O(D1) yield from dissociative ionization originating from molecular singlet oxygen [O2(Δg1)] are identified as well. Possible applications of the 2s22p4(D1)→2s12p5(P1) transition as a state-selective and sensitive probe of excited oxygen in combination with photoionization mass spectrometry are briefly discussed.