Multiphoton ionization of molecules

When they exposed a surface sample of organic molecules to intense UV radiation pulses, Antonov et al. (1980a) observed the formation of molecular ions without any noticeable fragmentation, which had the character of a nonthermal process. A detailed investigation led to an understanding of the two-stage character of the process in the form of thermal desorption of the molecules and their subsequent photoionization. A study of the laser thermal-desorption (ablation) process revealed the possibility of soft chemoionization in the dense cloud of laser-desorbed molecules (Karas and Hillenkamp 1987). This process was called matrix-assisted laser desorption/ionization 

As in the case of resonance photoionization of atoms (Chapter 9), the photoionization of molecules is carried out under various conditions (in a thermal molecular beam, a pulsed jet-cooled molecular beam, or a laser-desorbed molecular cloud). But in all cases, use is made of mass spectrometry of the photoions produced, for the combination of the optical and mass spectra makes the method much more informative. All these methods for laser ionization and laser desorption/ionization of molecules are briefly discussed below. 

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The overlap of molecular spectra with atomic lines, which occurs in optical flame spectrometry, has been less commonly encountered with LEI. Native flame species such as OH and CH are not observed because of their high ionization potentials. Resonantly-enhanced multiphoton ionization of molecules such as NO 45,46) may cause interferences in some flames. The LEI spectra of oxides of lanthanum, scandium,... 

Similarly, intense lasers make it possible to study the early stages in the multiphoton ionization of molecules through multiply excited states. Above-threshold ionization gives rise to a quasi-free electronic wavepacket state, and the recombination (collision) of such an electronic wavepacket with the remaining cation species results in the high harmonic... 

Fig. 10.1 Energy-level diagram of multiphoton ionization of molecules (a) resonance two-photon ionization (b) resonance-enhanced multiphoton ionization (REMPI) and (c) nonresonance multiphoton ionization (MPI).

An experiment of this kind was performed (Bagratashvili et al. 1983) with anthracene (C14H10) molecules (5 = 66, Dq = 4.8 eV), for which the estimate in eqn (10.1) gives — Eg c 3Eg. With so strong an overexcitation, ionization of the molecule (I = 7.4 eV) is quite possible. In fact, the formation of anthracene molecular ions was observed when the molecules were irradiated by sufficiently powerful (about 10 W/cm ) short (70 ns) C02-laser pulses under collisionless conditions (pressure 4 X 10 Torr) in a time-of-flight mass spectrometer. These ions can be believed to appear as a result of the IR multiphoton ionization of molecules in accordance with the scheme of Fig. 10.5(b). Of course, such a multiphoton ionization technique is applicable only to large polyatomic molecules, since for polyatomic molecules with a small number of atoms the maximum possible degree of overexcitation above the dissociation threshold is comparatively low. 

L. Zandee, R. B. Bernstein, and D. A. Lichtin. Vibronic/Mass Spectroscopy via Multiphoton Ionization of a Molecular Beam The I2 Molecule. J. Chem. Phys., 69(1978) 3427-3429. 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

Reisler, H. and Wittig, C. (1985). Multiphoton ionization of gaseous molecules, Adv. Chem. Phys. 60, 1. 

Upon multiphoton ionization of the neutral ammonia clusters, protonated ions are formed via the following internal ion-molecule reactions (Echt et al. 1985 Morgan and Castleman 1989 Wei et al. 1990a,b) ... 

Although the peak power of the pump laser must be high, the power of the probe laser should be kept low to avoid nonlinear effects (multiphoton absorption, stimulated Raman scattering see Section 3.9.2) and dielectric breakdown (ionization of molecules) that damage the sample. Thus, signal averaging of many pulses (high repetition rate) is made to obtain acceptable S/N ratios. Multichannel detectors such as an intensified silicon photoiode... 

The nonlinear phenomena in intense magnetic and laser fields are given some prominence in this volume. Two chapters deal with multiphoton processes and time-dependent phenomena in atoms. In another chapter it is emphasized that the process of multi-electron dissociative ionization of molecules offers considerable challenges both for modeling and for the study of first-principles. The dynamics of molecules in such intense laser fields is an area of great interest, both at the time of writing and for future studies. In all these chapters the interplay between theory and experiment is demonstrated. 

Excitation of atoms or molecules may be followed by collisional or multistep (multiphoton) ionization of the excited species. The charged particles so formed may be recorded by conventional methods. Several teehniques involve a combination of optical excitation of a neutral particle and its subsequent ionization. 

Ionization of molecules of neat liquids or of solutions can also be achieved by multiphoton absorption. Generally, the sum of the photon energies should exceed the ionization energy, 1 or Isoiute(liq)/ respectively. Two cases have to be distinguished ... 

Faidas, H. and Christophorou, L. G., Laser multiphoton ionization of aromatic molecules in nonpolar liquids, Radiat. Phys. Chem., 32, 433,1988. 

LASER MULTIPHOTON IONIZATION OF AROMATIC MOLECULES IN NONPOLAR LIQUIDS ... 

Successful experiments on the resonance-enhanced multiphoton ionization of polyatomic molecules by intense UV laser pulses with a tunable frequency were performed by Zandee and Bernstein (1979). Such experiments commonly use a time-of-flight... 

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. 

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. 

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. 

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