Laser resonance photoion

Comparison of Characteristics of Field-Ion Microscopy (FIM) and Laser Resonance Photoion (Electron) Spectromicroscopy (LRFSM)... 

S.V. Andreev, V.S. Letokhov, V.I. Mishin Laser resonance photoionization spectroscopy of Rydberg levels in Fr. Phys. Rev. Lett. 59, 1274 (1987)... 

Photoionization detection in a buffer gas has also been used to study the properties of superheavy (transuranium) elements with charge numbers Z > 92. Isotopes of such elements can only be produced by fission reactions in heavy-ion collisions or by transfer reactions using radioactive targets. The elements produced can be placed in an optical buffer-gas cell for the purpose of laser resonance photoionization spectroscopy. This was successfully demonstrated with atoms of such radioactive elements as americium (Z = 95) (Backe et al. 2000), einsteinium (Z = 99) (Kohler et al. 1997), and fermium (Z = 100) (Sewtz et al. 2003). 

In resonance photoionization of a neutral particle (atom or molecule), the absorption by the particle of a few laser photons gives rise to an easily detectable pair of charged particles—a photoion and a photoelectron. In most experiments, the number of charged particles... 

The magnification attended in the experiment with the photoelectron microscope was M = 10s, and the spatial resolution was around 30 nm, which proved sufficient for the visualization of individual color centers in a LiF crystal with the concentration of such centers less than 10l7cm 3. The results obtained in Ref. 9 may be considered the first successful implementation of laser resonance photoelectron microscopy possessing both subwavelength spatial resolution and chemical selectivity (spectral resolution). It will be necessary to increase the spatial resolution of the technique by approximately an order of magnitude and substantially improve its spectral resolution by effecting resonance multistep photoionization by means of tunable ultrashort laser pulses. 

In conclusion I would like to emphasize that the suggested approach (femtosecond laser spectromicroscopy) is not a simple modification of the Muller microscope [6], for the electric field here is not the decisive factor but serves solely to form the image. Table I lists the comparative characteristics of the Muller projection field-ion microscope (FIM) and proposed laser resonance photoelectron (photoion) spectromicroscope (LRFSM). 

Laser spectroscopy of the 1S-2S transition has been performed by Mills and coworkers at Bell Laboratories (Chu, Mills and Hall, 1984 Fee et al, 1993a, b) following the first excitation of this transition by Chu and Mills (1982). Apart from various technicalities, the main difference between the 1984 and 1993 measurements was that in the latter a pulse created from a tuned 486 nm continuous-wave laser with a Fabry-Perot power build-up cavity, was used to excite the transition by two-photon Doppler-free absorption, followed by photoionization from the 2S level using an intense pulsed YAG laser doubled to 532 nm. Chu, Mills and Hall (1984), however, employed an intense pulsed 486 nm laser to photoionize the positronium directly by three-photon absorption from the ground state in tuning through the resonance. For reasons outlined by Fee et al. (1993b), it was hoped that the use of a continuous-wave laser to excite the transition would lead to a more accurate determination of the frequency interval than the value 1233 607 218.9 10.7 MHz obtained in the pulsed 486 nm laser experiment (after correction by Danzmann, Fee and Chu, 1989, and adjustment consequent on a recalibration of the Te2 reference line by McIntyre and Hansch, 1986). 

In this section we outline a theoretical framework enabling the 2 + 1 resonant photoionization of a hydrogenic atom to be analyzed. Generally, the process is assumed to be induced by a non-monochromatic laser field with a time-dependent amplitude and taking place in the presence of intermediate resonance. It is shown... 

Dissociation and ionization by means of one or several lasers. Sensitive detection of fragments and ions with resonance photoionization and mass spectrometric techniques (tunable lasers). Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) with pulsed ablating laser sources. High lateral resolution enables molecular microprobing of biological cell compounds... 

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. 

It is quite natural to carry out the resonance photoionization of molecules via their electronically excited states. But this is not the only possibility. Polyatomic molecules can also be excited via their highly excited vibrational states (Bagratashvili et al. 1983). The former approach has found the widest application (Lubman 1990), thanks to the availability of high-power U V lasers, whereas the latter is not so widespread, as powerful tunable IR lasers are scarce. However, both these methods deserve a brief consideration. 

To achieve highly selective and sensitive detection of molecules, it was suggested that one should use laser resonance excitation of molecular vibrations, followed by photoionization of the vibrationaUy excited molecules, that is, vibrationally mediated photoionization (Ambartzumian and Letokhov 1972). As a matter of fact, it proved very difficult to realize this idea because the shift of the VUV absorption bands of the molecules caused by their vibrational excitation was too small. However, there exists another possibility for polyatomic molecules, namely multiphoton (MP) excitation of high-lying vibrational states by high-power IR laser pulses timed to resonance with the pertinent vibrational transitions (Chapter 11). The highly excited vibrational states can be photoionized by another UV or VUV laser pulse. This molecular-photoionization experiment was performed in accordance with the scheme... 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

Photoionization, where electrons are released by molecules following the absorption of energy from photons, has long been viewed as a non-radioactive means to ionize explosives in the vapor phase [39]. In recent years, two teams have sought to employ laser ionization with IMS for explosive determinations. A team at Implant Sciences Corporation has utilized a laser (or flash lamp) for sampling surfaces and for ionization of sample vapors in an IMS analyzer [40, 41]. In their approach, the sample is removed from a surface with an increased temperature from laser exposure. Gases (and presumably particulate matter) from over the surface are drawn into an IMS drift tube using a wall-free inlet vida supra). In the IMS drift tube, resonance multi-photon ionization by a laser is used to produce ions from the explosives. Their... 

In practice, for application to ambient air, efficient photoionization requires the use of pulsed lasers and multiphoton absorption methods. The terms multiphoton ionization, or MPI, and resonance-enhanced multiphoton ionization, or REMPI, are used to describe these processes. 

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