Multiphoton

Baumert T, Engel V, Meier Ch and Gerber G 1992 High laser field effects in multiphoton ionization of Na2 -experiment and quantum calculations Chem. Rhys. Lett. 200 488... 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Quack M 1998 Multiphoton excitation Encyclopedia of Computational Chemistry vol 3, ed P v R Schleyer et al (New York Wiley) pp 1775-91... 

Figure A3.13.2. Illustration of the analysis of the master equation in temis of its eigenvalues and example of IR-multiphoton excitation. The dashed lines give the long time straight line luniting behaviour. The fiill line to the right-hand side is for v = F (t) with a straight line of slope The intercept of the...

Figure A3.13.12. Evolution of the probability for a right-handed ehiral stmetnre (fiill eiirve, see ( equation (A3,13.69))) of the CH eliromophore in CHD2T (a) and CHDT2 ( ) after preparation of ehiral stnietures with multiphoton laser exeitation, as diseussed in the text (see also [154]). For eomparison, the time evolution of aeeording to a one-dimensional model ineluding only the bending mode (dashed enrve) is also shown. The left-hand side insert shows the time evolution of within the one-dimensional ealeulations for a longer time interval the right-hand insert shows the time evolution within the tln-ee-dimensional ealeulation for the same time interval (see text).

Wyatt R E, Hose G and Taylor H S 1983 Mode-selective multiphoton excitation in a model system Phys. Rev. A 28 815-28... 

Quack M and Sutcliffe E 1983 Quantum interference in the IR-multiphoton excitation of small asymmetric-top molecules ozone Chem. Phys. Lett. 99 167-72... 

Herve S, Le Quere F and Marquardt R 2001 Rotational and vibrational wave packet motion during the IR multiphoton excitation of HF J. Chem. Phys. 114 826-35... 

Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

All the previous discussion in this chapter has been concerned with absorption or emission of a single photon. However, it is possible for an atom or molecule to absorb two or more photons simultaneously from a light beam to produce an excited state whose energy is the sum of the energies of the photons absorbed. This can happen even when there is no intemrediate stationary state of the system at the energy of one of the photons. The possibility was first demonstrated theoretically by Maria Goppert-Mayer in 1931 [29], but experimental observations had to await the development of the laser. Multiphoton spectroscopy is now a iisefiil technique [30, 31]. 

For molecules exposed to the intensity of sunlight at the earth s surface this would suggest that the molecule might be excited once in the age of the universe. However, the probability is proportional to the square of the light intensity. For a molecule exposed to a pulsed laser focused to a small spot, the probability of being excited by one pulse may be easily observable by fluorescence excitation or multiphoton ionization teclnhques. 

Three-photon absorption has also been observed by multiphoton ionization, giving Rydberg states of atoms or molecules [36]. Such states usually require vacuum ultraviolet teclmiques for one-photon spectra, but can be done with a visible or near-ultraviolet laser by tluee-photon absorption. 

Ashfold M N R and Howe J D 1994 Multiphoton spectroscopy of molecular species Ann. Rev. Phys. Chem. 45 57-82... 

Johnson P M 1976 The multiphoton ionization spectrum of benzene J. Chem. Rhys. 64 4143-8... 

Wight C A and Armentrout P B 1993 Laser photoionization probes of ligand-binding effects in multiphoton dissociation of gas-phase transition-metal complexes ACS Symposium Series 530 61-74... 

Belbruno J J 1995 Multiphoton ionization and chemical-dynamics Int. Rev. Phys. Chem. 14 67-84... 

For > 0, one has absorption for < 0, emission. Multiphoton absorption and emission fall into this class. The Class I Raman spectroscopies clearly exliibit a net absorption of energy in Stokes scattering and a... 

Nesselrodt D R, Potts A R and Baer T 1995 Stereochemical analysis of methyl-substituted cyclohexanes using 2+1 resonance enhanced multiphoton ionization Anal. Chem. 67 4322-9... 

B1.18.5.6 CONFOCAL MICROSCOPY WITH MULTIPHOTON-EXCITATION FLUORESCENCE... 

One-photon excitation has lunitations due to the unwanted out-of-focus fliiorophore absorption and bleaching, and light scattering. These drawbacks can be circumvented if multiphoton excitation of the fliiorophore is used. Since it increases with the nth power of the photon density, significant absorption of the exciting light will only occur at the focal point of the objective where the required high photon density for absorption is reached. Consequently, only... 

Thus, multiphoton excitation eliminates unwanted out-of-focus excitation, lumecessary phototoxity and bleaching. However, efficient power sources are required and, since the efficiency of multiphoton excitation is usually low, the times needed to generate unages are mcreased. 

Wokosin D L, Centonze V, White J G, Armstrong D, Robertson G and Ferguson A I 1996 All-solid-state ultrafast lasers facilitate multiphoton excitation fluorescence imaging IEEE J. Sel. Top. Quantum Electron. 21051-65... 

Figure B2.3.8. Energy-level sehemes deseribing various optieal methods for state-seleetively deteeting ehemieal reaetion produets left-hand side, laser-indueed fluoreseenee (LIF) eentre, resonanee-enlianeed multiphoton ionization (REMPI) and right-hand side, eoherent anti-Stokes Raman speetroseopy (CARS). The ionization oontinuiim is denoted by a shaded area. The dashed lines indieate virtual eleetronie states. Straight arrows indieate eoherent radiation, while a wavy arrow denotes spontaneous emission.

The most widely used of these tecluiiques is resonance-enlianced multiphoton ionization (REMPI) [ ]. A schematic energy-level diagram of the most conunonly employed variant (2 + 1) of this detection scheme is illustrated in the... 

Dagdigian P J, Varley D F, Liyanage R, Gordon R J and Field R W 1996 Detection of DCI by multiphoton ionization and determination of DCI and HCI internal state distributions J. Chem. Phys. 106 10 251-62... 

This technique with very high frequency resolution was used to study the population of different hyperfme structure levels of the iodine atom produced by the IR-laser-flash photolysis of organic iodides tluough multiphoton excitation ... 

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

The record m the number of absorbed photons (about 500 photons of a CO2 laser) was reached with the CgQ molecule [77]. This case proved an exception in that the primary reaction was ionization. The IR multiphoton excitation is the starting pomt for a new gas-phase photochemistry, IR laser chemistry, which encompasses numerous chemical processes. 

B) The multiphoton excitation of electronic levels of atoms and molecules with visible or UV radiation generally leads to ionization. The mechanism is generally a combination of direct, Goeppert-Mayer, and quasi-resonant stepwise processes. Since ionization often requires only two or tln-ee photons, this type of multiphoton excitation is used for spectroscopic purposes in combination with mass-spectrometric detection of ions. 

---