Multiphoton Ionization Processes in

In Sect. 3.1.1, for a similar system, the model molecule K2 excited to its A state, the wave packet propagation is explored in greater detail by real-time three-photon ionization (3PI) spectroscopy. Applying laser pulses of moderate intensities allows the selective detection of the pure vibrations of the A state, in excellent agreement with quantum dynamical calculations [42]. Both the favorable spectroscopic properties of K2 and the special molecular dynamics induced by the selected moderate laser intensity combine to open 

Ever since the development of the laser, the dream of using light to control the future of matter has been of extraordinary interest to physicists and chemists. With the femtosecond laser technique this dream has come 

The first examples of experiments of this type [19, 29, 30, 37, 66, 67] have proved the ideas of Tannor, Rice, and Kosloff [60, 61]. Most of these pump-fecontrol experiments were carried out on diatomic molecules. In larger systems with three or more vibrational degrees of freedom, the situation becomes much more complicated and it is a fascinating question whether the concept of controlled molecular dynamics can still be realized (see Sect. 3.2.4). 

Two examples which emerged from a fruitful cooperation of theory and experiment are discussed in this book. First, once again the K2 molecule excited to its A state acts as a model system (Sect. 3.1.5). Its special spectroscopic properties combined with the dynamics induced by femtosecond state preparation facilitate the transition from pump probe to control spectroscopy. The intensity of the pump pulse serves as the control parameter, allowing the forcing of the molecule to perform either the A state vibrations or its ground state dynamics. 

The general complementarity of sensitivities in cw and femtosecond spectroscopy has been anticipated by Zewail [66] and it is verified for the Naa molecule excited to its electronic B state (see Sect. 3.2.4). This system has already been studied in great detail by various experimental and theoretical techniques such as cw two-photon ionization spectroscopy [68-70], femtosecond pump probe spectroscopy at high intensities [29, 30, 71], quantum ab initio studies [72-74], two-dimensional simulations of the pseudorot at ional progressions in the cw absorption spectra [75-78], and, finally, three-dimensional simulations by means of empirical potential-energy surfaces (PESs) [79, 80]. 

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Figure 24. Coincidence-imaging spectroscopy of dissociative multiphoton ionization processes in NO2 with 100-fs laser pulses at 375.3 nm, using angle-angle correlations. The polar plots show, at time delays of Ofs, 350 fs, 500 fs, 1 ps, and 10 ps, the angular correlation between the ejected electron and NO photofragment when the latter is ejected parallel to the laser field polarization vector. The intensity distributions change from a forward-backward asymmetric distribution at early times to a symmetric angular distribution at later times, yielding detailed information about the molecule as it dissociates. Taken with permission from Ref. [137]...

With this short overview of molecular wave packet dynamics in mind, the wave packet propagation in small prototype molecules will now be examined more concretely. Different theoretical methods exist to describe ultrafast multiphoton ionization processes in diatomics and have been discussed in detail in previous work [3, 33-35, 291-294]. The theoretical approach used here is adapted specially to the presented 3d problems and was improved in Manz s group, mainly by de Vivie-Riedle [295] assisted by Reischl [81]. A few special features of their theoretical ansatz applied to the pumpfeprobe experiments carried out on the model molecules K2 and Naa are now briefly summarized. 

For the two-color and one-color experiments the values of k are 0.08 and 0.47 respectively. The rather large value of k for the one-color experiment can be explained by the multiphoton ionization process in the probe step [42]. A wave packet prepared by the pump pulse on the A 17+ potential-... 

Multiphoton Ionization Processes in K2 from Pump Probe to Control... 

Fig. 3.28. Snapshots of the wave packets in their respective electronic states for a delay time of 300 fs between the pump and probe lasers for the (direct) (1+2) multiphoton ionization process in K2 (moderate laser field). The wave packets are represented by their absolute values. The two-photon step is indicated by the arrows. The 2 /7g state is included in the dynamical calculation the state is...

R. de Vivie-Riedle, B. Reischl, S. Rutz, and E. Schreiber, Femtosecond Study of Multiphoton Ionization Processes in K2 at Moderate Lciser Intensities , J. Phys. Chem. 99, 16829 (1995). 

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Fig. 2. Energy diagrams schematically representing photoionization processes from the atomic ground state (a) single-photon ionization resulting from the absorption of one high-frequency UV photon (here it is the 13th harmonic of an IR laser) (b) ATI multiphoton ionization occurring in the presence of an intense IR laser (c) two-colour ionization occurring in the simultaneous presence of the IR laser and of the UV harmonic...

How many photons of 650-nm wavelength are required to ionize cyclopentene in a multiphoton ionization process ... 

Photoelectron spectroscopy can also be carried out by measuring the distribution of flight times of photoemitted electrons. This method is useful in systems in which the absorbing species are produced by a pulsed source and the photolysis radiation is also pulsed. These electron time of flight methods have been used to elucidate structures of transient species such as free radicals and clusters produced in pulsed photolysis sources and in assessing the vibrational-state purity of ions produced in multiphoton ionization processes, particularly those in which the final photon absorption process is from a Rydberg state whose geometry is similar to that of the ion. 

The above techniques are restricted to molecttles which fluoresce. A more general technique is multiphoton ionization (MPI) in which a molecule absorbs several photons sufficient in energy to produce a molecttlar ion. This technique is very sensitive since single ions can be detected. The process may involve a single laser and several photons, or two (or more) lasers with various corribinations of photons, e.g. 1+3, 2+2. One of the lasers can be adjusted so as to involve an intermediate excited state in which case the sensitivity is considerably enhanced and the process is known as resonance enhanced multiphoton ionization (REMPI). The detection of the resrtltant ion with a mass-spectrometer further refines the specificity of the method, and allows individual mass peaks, and isotopic species, to be monitored. The introduction of ZEKE (zero electron kinetic energy [42]) considerably increases the resolution which is beginning to approach the hmit imposed by the widths of the laser. 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

Laser ionization. Occurs when a sample is irradiated with a laser beam. In the irradiation of gaseous samples, ionization occurs via a single- or multiphoton process. In the case of solid samples, ionization occurs via a thermal process. 

The advancement of the application of lasers in combination with the molecular beam technique has made a great impact in the understanding of primary photodissociation processes. For state-specific detection of small fragments, laser-induced fluorescence, multiphoton ionization, and coherent laser scattering have provided extremely detailed information on the dynamics of photodissociation. Unfortunately, a large number of interesting... 

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