Multiphoton processes spectroscopy

S. I. Chu, Advances in Multiphoton Processes and Spectroscopy, World Scientific, Singapore, 2986, vol. 2. 

Gases - [AIRPOLLUTION CONTROLMETHODS] (Vol 1) - [COAL CONVERSION PROCESSES - GASIFICATION] (Void) -m air pollution models [ATMOSPITERIC MODELING] (Vol 3) -analysis by multiphoton ionization [SPECTROSCOPY, OPTICAL] (Vol 22) -as lubricants [LUBRICATION AND LUBRICANTS] (Vol 15) -sampling of [SAMPLING] (Vol 21) -sterile filtration [MICROBIAL AND VIRAL FILTRATION] (Supplement)... 

Atomic cryocrystals which are widely used as inert matrices in the matrix isolated spectroscopy become non-inert after excitation of an electronic subsystem. Local elastic and inelastic lattice deformation around trapped electronic excitations, population of antibonding electronic states during relaxation of the molecular-like centers, and excitation of the Rydberg states of guest species are the moving force of Frenkel-pairs formation in the bulk and desorption of atoms and molecules from the surface of the condensed rare gases. Even a tiny probability of exciton or electron-hole pair creation in the multiphoton processes under, e.g., laser irradiation has to be taken into account as it may considerably alter the energy relaxation pathways. 

Gordon, R. J., andFujimura, Y., eds. (2001). Advances in Multiphoton Processes and Spectroscopy, Volume 14. Quantum Control of Molecular Reaction Dynamics Proceedings of the US—Japan Workshop, World Scientific, Singapore. 

S. L. Chin, From Multiphoton to Tunnel Ionization, in Advances in Multiphoton Processes and Spectroscopy, eds. S. H. Lin, A. A. Villaeys and Y. Fujimura, World Scientific, Singapore, 16, 249-272 (2004)... 

The small cross sections in multiphoton processes are of course a weakness of nonlinear spectroscopy. Especially in microscopy, this problem becomes serious because of the small volume of a sample. By the use of the signal enhancement techniques, however, the disadvantage can be turned into an advantage of back-ground-free selective measurements. For example, the combined use of HRS with the plasmonic enhancement provides us a chemical imaging with nanoscale spatial resolution when a laser-illuminated metal tip is located adjacent to a sample surface, signal enhancement is locally induced near the tip. This spatial resolution is expected to overcome optical diffraction limit. Such tip-enhanced spectroscopy has already been reported in conventional CARS [15]. 

That lasers have played a key role as promoters and as probes of chemical reactions is well known and extensively documented.1,4,7,62-72 In many of these applications the laser is employed as an intense, nearly monochromatic, light source whose characteristics ensure species selectivity, a well-characterized spectroscopy, and adequate intensity for multiphoton processes. Some possible applications, notably laser-assisted collisions 73,74 and transition-state spectroscopy,75,76 are yet in their infancy, but the extant studies already suggest considerable promise for influencing and probing chemical reactions. 

Multiphoton Laser Spectroscopy in Heavy Elements", in "Multiphoton Processes," Eberly, J. H. and Lambropoulos,... 

The subject of multiphoton excitation spectroscopy began in 1931 when Goppert-Mayer [450] wrote a theoretical paper in which she calculated the transition rate for an atom in the presence of two photons rather than just one. At the time, the process seemed rather exotic, and it was reassuring that the calculated rate was so low as to guarantee that it could not readily be observed in the laboratory with conventional sources. This conclusion was reassuring because it implies that a simple perturbative theory (one photon per transition is the weak-field limit) is adequate for most purposes. 

The term action spectroscopy refers to those techniques that do not directly measure the absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [42], multiphoton ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [50, 51], vibrational predissociation spectroscopy [52] and optothermal spectroscopy [53, M]- These techniques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

G. MUller, K.L. Kompa, J.L. Lyman, W.E. Schmid, S. Trushin, 2-Step Photoionization of Benzene Mechanism and Spectroscopy in Multiphoton Processes, P. Lambropoulos, S.J. Smith, Eds. Springer Series on Atoms and Plasmas 1984. 

Laser-microwave spectroscopy based on nonlinear phenomena developed from the type of experiments on molecules already discussed in Section 3.2 which make use of optical pumping or double resonance. Occasionally, the laser and the rf power were high enough to create the nonlinear phenomena mentioned above, i.e., to saturate the transitions involved and/or to induce multiphoton transitions. The intermediate level in, e.g., two-photon transitions did not have to be a real state but could be virtual as well. Therefore, a drawback often encountered in earlier infared laser-microwave experiments could be avoided if the laser transition frequency did not exactly coincide with the molecular absorption line the Stark or Zeeman effect had to be used for tuning. This results in an undesired line splitting. With laser-microwave multiphoton processes, however, the laser can be operated at its inherent transition frequency. Exact resonance with molecular lines is then achieved by using a nonlinear effect, i.e., a radiofrequency quantum is added to or subtracted from the laser frequency (see Figure 28). 

Spectroscopy and Time-of-Flight Analysis. Multiphoton techniques are now widely used to obtain information on bound states that cannot be obtained by singlephoton absorption spectroscopy. Because multiphoton processes are nonlinear, it is not practical to monitor the absorption of the laser beam intensity. Thus, a more common approach is to use laser induced fluorescence (LIF), where the absorption threshold for electronic states is obtained from the onset of fluorescence radiation. Alternatively (as we have indicated above) absorption spectra can be obtained by detecting ions produced by resonant ionization. 

Another important aspect about the optical properties of QDs is the multiphoton process which has been widely applied in recent years in biological and medical imaging after the pioneer work of Goeppert-Mayer (1931), Lami et al. (1996), Helmchen et al. (1996), Yokoyama et al. (2006). The multiphoton process has largely been treated theoretically by steady-state perturbation approaches, for example, the scaling rules of multiphoton absorption by Wherrett (1984) and the analysis of two-photon excitation spectroscopy of CdSe QDs by Schmidt et al. (1996). Non-perturbation time-dependent Schrodinger equation was solved to analyze the ultrafast (fs) and ultra-intense (in many experiments the optical power of laser pulse peak can reach... 

A technique which combines the high sensitivity of resonant laser ionization methods with the advantages of nonlinear coherent Raman spectroscopy is called IDSRS (ionization detected stimulated Raman spectroscopy). The excitation process, illustrated in Figure 5, can be briefly described as a two-step photoexcitation process followed by ion/electron detection. In the first step two intense narrow-band lasers (ct L, 0) ) are used to vibrationally excite the molecule via the stimulated Raman process. The excited molecules are then selectively ionized in a second step via a two- or multiphoton process. If there are intermediate resonant states involved (as state c in Figure 5), the method is called REMPI (resonance enhanced multi-photon ionization)-detected stimulated Raman spectroscopy. The technique allows an increase in sensitivity of over three orders of magnitude because ions can be detected with much higher sensitivity than photons. 

This may significantly reduce noise problems caused by detector noise or background radiation. Furthermore, the large intensity allows new nonlinear spectroscopic techniques such as saturation spectroscopy (Sect.10.2), or multiphoton processes (Sect.10.5), which open new possibilities of studying molecular transitions not accessible to linear spectroscopy. 

In this chapter we describe advances in the femtosecond time-resolved multiphoton photoemission spectroscopy (TR-MPP) as a method for probing electronic structure and ultrafast interfacial charge transfer dynamics of adsorbate-covered solid surfaces. The focus is on surface science-based approaches that combine ultrafast optical pump probe excitation to induce nonlinear multi-photon photoemission (MPP) from clean or adsorbate covered single crystal surfaces. The photoemitted electrons transmit spectroscopic and dynamical information, which is captured by their energy analysis in real or reciprocal space. We examine how photoelectron spectroscopy and microscopy yield information on the unoccupied molecular structure, electron transfer and relaxation processes, light induced chemical and physical transformations and the evolution of coherent single particle and collective excitations at solid surfaces. 

---