One-photon ionization

Figure 9. Phase lag spectrum of HI and DI in the vicinity of the 5sa resonance. The top panel shows the phase lag between photoionization and photodissociation of HI (filled circles) and DI (open circles), the phase lag between the photoionization of HI and DI (squares), and the phase lag between the photoionization of HI and H2S (triangles). The bottom two panels show the one-photon ionization spectra of HI and H2S. (Reproduced with permission from Ref. 30, Copyright 1999 American Physical Society.)...

The pump-probe scheme that we use is as follows the molecule is initially excited by a pump laser photon (htOj) to the first excited neutral state. The dynamics is then followed with a probe laser by one-photon ionization (hooa). Compared to a free diatomic, solvation by CH3CN brings two new factors come into play. Firstly, the potential energy curves of the diatomic are modified by the presence of the neighboring molecule. Secondly, the fragmentation dynamics of the diatomic is changed as there may be collisions with, and a transfer of energy to, the acetonitrile. 

The reason for the weakness of the PFI spectrum is of interest in itself, in that the one-photon ionization from the B 2II state to the ground state of the ion is formally forbidden as it involves a nominal two-electron change ... 

Yakhontov in this volume). The analytic relations obtained in the latter work admit error-free numerical evaluation at values of Wl lying both below and above respective ionization thresholds. Note that in the latter case Q ea 071,2) becomes complex-valued, with its imaginary part being proportional to the width of the state due to the one-photon ionization. 

In order to check our numerical calculations we need analytical results. Let us assume that the surface state electrons are exposed to a weak homogeneous cw microwave field characterized by f t) = —esin(ci <), where e is the strength of the microwave field. In this case it is possible to calculate one-photon ionization rates analytically. We start with the amplitude equations (6.2.16) and (6.2.17). Neglecting the continuum-... 

This result can be compared with the one-photon ionization rates derived by Casati et al. (1987)... 

For u > 1/2 direct one-photon ionization from the ground state to the continuum is possible. We denote this process by 1 —> C, where C stands for continuum . At lower frequencies, an infinity of two-step processes is possible according to 1 n C, n = 2,3,. For every n we expect to see an associated peak in the ionization probabiUty. The peaks are expected to occur at... 

Cm also appear. Spectrum b is similar, except that the Cio/Cyo peak ratio is reduced to 2.1. Therefore sample b, produced at the higher cell temperature, has six times more C70 relative to Cao than sample a. The mysterious long tails to the high mass sides of both the Cfto and C70 peaks are not yet understood, but they may be reaction products with air. The variation of the tail signals with ArF laser fluence suggests that these species are one-photon ionized and... 

A Time-of-Flight Mass Spectroscopic Study in Combination with One-Photon Ionization... 

Such a one-photon ionization was performed with the use of photons of 10.5 eV/photon (=118 nm), that is, the ninth harmonics of the fundamental radiation of a Nd YAG laser [14]. At this photon energy a considerable number of ablated products can be one-photon ionized. 

Using the same technique we have been studying TOF mass spectra of neutral products which are laser-ablated from graphite [15-18]. In our studies we mainly monitored the change of TOF mass spectra as a function of the time interval between the ablation laser pulse and the one-photon ionizing laser pulse. This time interval will be called, synonymously, delay time , for convenience. The observed dependence of the mass spectra on the delay time has provided us with information on the formation and stability of some products. 

An example of the difference in the effect of one-photon ionization (1 PI) and MPI can be seen from the two mass spectra in Figure 9.2, which were obtained by using the ninth harmonics of a Nd YAG laser (118 run = 10.5eV/photon) for IPI and by using the third harmonics of a Nd YAG laser (355nm = 3.5eV/photon) for MPI in the Lie buffer gas. 

FIGURE 9.2 Comparison of TOF mass spectra obtained by one-photon ionization (IPI) and multi-photon ionization (MPI) of laser ablated graphite in the He buffer gas. 

As the bottom spectrum shows, two types of species, i.e. C ( > 6) and C I 1, ( > 4, m = 1-4) were observed whose ionization energies are lower than the photon energy of 10.5 eV. The absence of C4, C5, and C4H, whose ionization energies exceed the photon energy of 10.5 eV, supports the presumption that in the present work only one-photon ionization, but not MPI, took place which is favorable for the study of ablated neutral products. 

FIGURE 9.4 Mass spectra of laser ablated graphite in the H, buffer gas. One-photon ionization with lO.SeV/photon. Delay times (gs) are shown to the right. Spectra observed at 55 and 65 gs are expanded by a factor of 20 and those at 200 and 8000 gs by 40. TOF mass peak assignment is given in the bottom spectrum. Vertical dotted lines are to assist reading the subtle but significant shitt of the peaks. 

Figure 8.20 PES of HC1 arising from the two-photon excitation 1 A2(v = 0, J = 2) <— X1 E+ ( " = 0, J" = 2), followed by one-photon ionization to the 2II ionic ground state (from...

In order to compare ionization efficiencies of two-photon and one-photon ionization we investigated an aromatic polymer that can readily be ionized by two-photon ionization at 193 nm, namely polystyrene. The sample composition corresponds to the structure C4H9—(CH2—CH0) —H, where 0 = phenyl. Figure 11.7 shows representative mass spectra of samples with nominal Mp... 

Resonantly enhanced n photon PES is often best described as the preparation of an excited neutral state by (n-l)-photons, followed by one-photon ionization. Thus, the PES will contain the same information as conventional one-photon PES except that the "initial state in the ionization event will be an electronic excited state rather than the ground state. Thus, the structure of the n-photon PES will reflect the difference between the excited neutral state and the states of the ion. The use of pulsed laser ionization creates the electrons at a well defined point in time and space. This makes it possible to design relatively simple spectrometers which measure the time-of-flight TOF for the electron to travel from the point of ionization to the detector. 

This has an advantage over conventional, one-photon ionization methods which use rare gas resonance light sources. These are continuous sources and are used without focusing so that they irradiate a larger sample volume. However, there is a disadvantage to creating the electrons in short bursts. As the first electrons fly away from the... 

Fig. 1.36 Level schemes of ionization spectroscopy (a) photoionization (b) excitation of autoion-izing Rydberg levels (c) two-photon ionization of excited molecules (d) one-photon ionization of a high lying level, excited by non-resonant two-photon process (e) three-photon excitation of a level which is ionized by a fourth photon (f) non-resonant two-photon ionization...

When an atom or molecule interacts with a photon of sufficient energy, ionization may occur through removal of an electron. Because the ionization potential of most small molecules is larger than 8 eV, highly energetic photons in the VUV are required to induce ionization through one-photon absorption. Tuneable radiation may be provided by synchrotron sources and by (normally very inefficient) non-linear conversion of radiation from visible or UV lasers. However, direct one-photon ionization exhibits little state selectivity, i.e. ionization of normally several vibrational and rotational levels of the electronic ground state occurs. Consequently, in the photo-ion one also encounters a superposition of several vibrational and rotational levels (determined mostly by the Franck-Condon factors between the initial and final vibrational states). 

This scheme reminds me to some extent of a very early idea about the photoionization of atoms in coherent states (Letokhov 1966). The idea considered was the one-photon ionization of an atom in a state composed of a superposition of two states with close energies Ei and E2, prepared by the action of an electromagnetic field of frequency u>2i = (E2 — Ei)/H. The coherent oscillations of the polarization at the frequency W21 have the result that the probability of photoionization of... 

Italics indicates states that are forbidden by a one-photon ionization from the neutral ground state. 

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