Molecules resonance ionization spectroscopy

Once the charged particles, ions and electrons, are formed they can be extracted out of the laser beam-molecule interaction volume and monitored using a charge-sensitive detector. The general principle to implement (resonant) ionization spectroscopy is shown in Figure 9.1. 

James A M, Kowaiczyk P, Langlois E, Campbell M D, Ogawa A and Simard B 1994 Resonant two photon ionization spectroscopy of the molecules Nb, and Nb2 J. Chem. Rhys. 101 4485... 

Very primary events in the chemical effect of radiations on matter are excitation and ionization of molecules, which result in the formation of neutral free radicals and radical ions. These reactive species play vital roles in the radiation-induced chemical reactions. As they are paramagnetic with an unpaired electron, electron spin resonance (ESR) spectroscopy has been a useful method for elucidating the mechanism of radiation-induced reactions in solid matter where radical species can be trapped temporarily. Since the early days of the chemical application of ESR, this method has been applied very often to the identification and quantification of free radicals in polymers irradiated by radiation [1]. This is probably because, from the view-point of fundamental research, a variety of free radicals are readily trapped in solid polymers and, from the view-point of applied research, these free radicals have close correlation with radiation-induced crosslinking and degradation of polymers. 

Luminescence measurements can also be carried out by using a conventional stopped-flow apparatus. In additional, stopped-flow techniques are used in conjunction with NMR see Nuclear Magnetic Resonance (NMR) Spectroscopy of Inorganic/Organometallic Molecules), MS, and electrospray ionization mass spectrometry (ESI-MS). 

Since the energies of ionizing radiations greatly exceed the ionization potential of atoms and molecules, ions and electrons are produced. They have been observed in some cases at low temperature and are usually very short-lived at room temperature. They have been studied mainly by electron spin resonance, absorption spectroscopy and pulse radiolysis. Recombination of opposite charges on warming usually produces light emission known as thermoluminescence. 

Multiphoton ionization spectroscopy has been reviewed in two recent articles. An apparatus has been described for constant intensity multiphoton ionization spectroscopy. A sensitive molecular vapour detection system utilizing resonance-enhanced two-photon ionization has been used to monitor naphthalene to a limit of 5 x 10 molecules cm Excitation was achieved... 

Bog Boggis, S.A., Dyke, J.M., Tabrizchi, M., Richter, R. Resonance enhanced multiphoton ionization spectroscopy of the NCI molecule E Rydberg states studied by 2-photon excitation from the a state. Mol. Phys. 97 (1999) 81-92. 

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...

The following estimation illustrates the possible sensitivity of resonant two-photon ionization spectroscopy (Fig. 1.36a). Let Nk be the density of excited molecules in level Ek, Pki the probability per second that a molecule in level Ek is ionized by photons from laser L2 and /la = Nin aikiS.x (1.34) the number of photons absorbed per second on the transition Ei Ek.li Rk is the total relaxation rate of level Ek, besides the ionization rate (spontaneous transitions plus collision-induced deactivation) the signal rate in counts per second for the absorption path length Ax and for incident laser photons per second under steady state conditions is ... 

REMPI has turned into one of most-applied spectroscopic tools in studies of both the spectroscopy and reaction dynamics of small molecules in the gas phase. The procedure of REMPI comprises the combination of two consecutive steps. First, resonant m-photon excitation promotes a ground electronic state molecule to an excited (ro-)vibronic state. Then one additional photon (or, more seldom, n additional photons) is then absorbed and the molecule is ionized... 

Photoelectron spectroscopy is a powerful technique to study ionic and electronically excited levels of atoms and molecules. In the case of single photon excitation of cold molecules the photoelectron spectrum reflects the internal energy levels of the ionic system. Many experiments are performed via two photon ionization enhanced by a one-photon resonance (R2PE spectroscopy) in which transitions to intermediate electronic levels are accessed which strongly enhance the ion yield. Photoelectron spectroscopy of molecules inside superfluid helium droplets is of particular interest since the interaction of free electrons with liquid helium is known to be highly repulsive, so much so that the electrons form bubbles of about 34 A diameter. In this section, three recent photoelectron spectra will be discussed those of bare helium droplets, of Ags clusters and of single aniline molecules in helium droplets. 

Von Helden G, van Heijnsbergen D, Meijer G (2003) Resonant ionization using IR light a new tool to study the spectroscopy and dynamics of gas-phase molecules and clusters. J Phys Chem A 107 1671-1688... 

Lubman, D. M., and Li, Liang (1990). Resonant two-photon ionization spectroscopy of biological molecules in supersonic jets volatilized by pulsed laser desorption. In Lasers and Mass-Spectrometry (ed. D. M. Lubman), pp. 352-382. Oxford University Press, New York. 

The possibilities of molecular beam spectroscopy can be enhanced by allowing for spectrally resolved fluorescence detection or for resonant two-photon ionization in combination with a mass spectrometer. Such a molecular beam apparatus is shown in Fig.9.5. The photomultiplier PMl monitors the total fluorescence Ifi(Al) as a function of the laser wavelength Al (excitation spectrum. Sect. 6.3), whereas PM2 records the dispersed fluorescence spectrum excited at a fixed laser wavelength, where the laser is stabilized onto a selected molecular absorption line. In a second crossing point of the molecular beam with two laser beams within the ion chamber of a quadru-pole mass spectrometer the molecules are selectively excited by Laser LI and the excited molecules are ionized by L2. 

In the technique of multiphoton-ionization spectroscopy, two or more photons excite atoms or molecules from the ground state to an excited state which may be ionized by several methods (see Sect.8.2.5), e.g., field ionization, photoionization, collisional, or surface ionization. If the laser is tuned to multiphoton resonances, ionization signals are obtained if the upper level is ionized which can be, for instance, monitored with the setup shown in Fig.8.42. The ionization probe is a thin wire inserted into a pipe containing the atomic vapor. If the probe is negatively biased relative to the walls of the pipe, thermionic emission will lead to space-charge-limited current. Ions produced by the laser excitation partly neutralize the space charge, thereby allowing an increased electron current to flow (see Sect.8.2.4). 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

Infrared, ultraviolet, and nuclear magnetic resonance spectroscopies differ from mass spectrometry in that they are nondestructive and involve the interaction of molecules with electromagnetic energy rather than with an ionizing source. Before beginning a study of these techniques, however, let s briefly review the nature of radiant energy and the electromagnetic spectrum. 

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