Non-resonant multiphoton ionization

Figure 2.1 shows the ionization mechanisms for atoms in high intensity laser fields. Non-resonant multiphoton ionization (NRMPI) is expected at an irradiation intensity of around 1013 W cm 2. Optical field ionization (OFI), which comprises tunneling ionization (TI) and barrier suppression ionization (BSI), occurs at an intensity above 1014Wcm 2. The original Coulomb potential is distorted enough for the electron to either tunnel out through or escape over the barrier. The threshold intensity of BSI for atoms can be estimated by (2.1) [14] ... 

Figure 4. Non resonant multiphoton ionization mass spectrum of L-Arginine.

Becker CH and Gillen KT (1984) Surface analysis by non-resonant multiphoton ionization of desorbed or sputtered species. Analytical Chemistry 56 1671-1677. 

Kaesdorf S, Hartmann M, Schroder H, and Kompa KL (1992) Influence of laser parameters on the detection efficiency of sputtered neutrals mass spectrometry based on non-resonant multiphoton ionization. International Journal of Mass Spectrometry and Ion Processes 116 219-247. 

The combination of resonant laser excitation to an intermediate level and the subsequent mass analysis of the ion also makes the technique species selective. Therefore, one can distinguish REMPI spectra of systems that have the same mass, e.g. phenol-H2 and phenol-CO. On the other hand, care needs to be taken not to approach the REMPI experiment in a brute-force approach, as one may be tempted to increase weak signals by increasing the laser pulse energies. Non-resonant multiphoton ionization (MPI) processes may destroy the carefully adjusted species selectivity, as will be shown in the example of the REMPI investigation of CaH/CaD reaction products see Figure 9.5 below. 

Using Rydberg atoms and microwave fields it has been possible to observe virtually all one electron strong field phenomena. The attraction of these experiments is that they can be more controlled than most laser experiments, with the result that more quantitative information can be extracted. The insights gained from these experiments can be profitably transferred to optical experiments. To demonstrate the latter point we demonstrate that apparently non-resonant microwave ionization, in fact, occurs by resonant transitions through intermediate states. These experiments demonstrated clearly the power of Floquet analysis of such processes, and the ideas were subsequently applied to the analogous problem of laser multiphoton ionization. 

Rydberg atoms and microwave fields constitute an ideal system for the study of atom-strong field effects, and they have been used to explore the entire range of one electron phenomena [5]. Here we focus on an illustrative example, which has a clear parallel in laser experiments, a series of experiments which show that apparently non-resonant microwave ionization of nonhydronic atoms proceeds via a sequence of resonant microwave multiphoton transitions and that this process can be understood quantitatively using a Floquet, or dressed state approach. 

In the following section the experimental approach is briefly described. The initial observations of microwave ionization and the completely non-resonant picture initially used to describe it are then presented. Then microwave multiphoton transitions in a two level system analogous to the rate limiting step of microwave ionization are described both experimentally and theoretically. Experiments on this two level system with well controlled pulses of microwaves to show the applicability of an adiabatic Floquet theory to pulses are then described. We finally return to microwave ionization to see evidence for the resonant nature of the process. 

The spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

A major advance in the utility of laser spectroscopy came as a result of the development of multiphoton ionization MPI as a means of detection of multiphoton absorption by molecules [1]. The resonance encountered as the n-photon energy of a scanning laser becomes coincident with that of a molecular excited state is evidenced by a large increase in ionization rate. Since single ionization events can be detected with near unit efficiency, this results in a very sensitive means of detecting weak multiphoton absorption. MPI is a more widely applicable method than laser induced fluorescence since it can be used for non-emitting states. 

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