Resonant multiphoton ionization

We have also carried out preliminary experiments in which we have detected the laser desorption of ethylene, cyanogen, methanol, and benzene from the Pt(s)[7(111) x (100)] surface. These spectra are shown in Figure 9. In the experiments involving ethylene, cyanogen, and methanol only neutral species are desorbed. In the case of benzene we observe the molecular parent ion in the absence of the electron beam. We believe that this is due to resonance multiphoton ionization of the benzene by the laser after desorption (resonance multiphoton ionization of benzene is very efficient with 249 nm radiation). These spectra are in marked contrast to the results of SIMS experiments which produce a wide variety of complex metal-adsorbate cluster ions. In the case of ethylene, our experiments were performed at 140 K, and under these conditions ethylene is known to be a molecular x-bonded species on the surface. In SIMS under these conditions the predominant species is CH (15)t but in the laser desorption FTMS experiments neutral ethylene is the principal species detected at low laser power. 

Figure 2.1 shows the ionization mechanisms for atoms in high intensity laser fields. Non-resonant multiphoton ionization (NRMPI) is expected at an irradiation intensity of around 1013 W cm 2. Optical field ionization (OFI), which comprises tunneling ionization (TI) and barrier suppression ionization (BSI), occurs at an intensity above 1014Wcm 2. The original Coulomb potential is distorted enough for the electron to either tunnel out through or escape over the barrier. The threshold intensity of BSI for atoms can be estimated by (2.1) [14] ... 

Highly Accurate Theoretical Simulation of the Resonant Multiphoton Ionization Processes With Simplest Atoms... 

The photodissociation of bromobenzene in solution has been investigated with ultrafast transient absorption spectroscopy, following excitation at 266 nm. The main kinetic feature in acetonitrile was a 9 ps decay that was assigned to predissociation similar decays were observed in hexane, dichloromethane and tetrachloromethane. Laser-aligned iodobenzene have been photodissociated into phenyl radicals and iodine atoms with a 1.5 ps laser pulse at 266 nm, and the yield of iodine photoproducts detected by resonant multiphoton ionization." Significant yield enhancements were observed when the dissociation laser was polarized parallel instead of perpendicular to the alignment laser polarization. 

Various forms of radiation have been used to produce ions in sufficient quantitites to yield neutral products for subsequent analysis. In principle, it should be possible to use intense beams of UV below ionization threshold for this purpose. To date, however, efforts to collect neutrals from resonant multiphoton ionization (REMPI) have not succeeded. In one experiment, 1 mbar of gaseous -propyl phenyl ether was irradiated at room temperature with a 0.1 W beam of 266 nm ultraviolet (from an 800 Hz laser that gives 8 n pulses) concurrent with a 0.5 W beam at 532 nm. The beams were intense enough not only to ionize the ether in the mass spectrometer, but also to excite it so that it expels propene. After several hours of irradiation < 10% of the starting material remained. Production of carbon monoxide and acetylene (decomposition products of the phenoxy group) could be detected by infrared absorption spectroscopy, but the yield of neutral propene (as measured by NMR spectroscopy) was infinitesimal. 

Figure 4. Non resonant multiphoton ionization mass spectrum of L-Arginine.

For measurements of cluster-size distributions in cold molecular beams (Sect. 4.3), or for monitoring the mass distribution of laser-desorbed molecules from surfaces, these combined techniques of laser ionization and mass spectrometry are very useful [105, 106]. For the detection of rare isotopes in the presence of other much more abundant isotopes, the double discrimination of isotope-selective excitation by the first laser LI and the subsequent mass separation by the mass spectrometer is essential to completely separate the isotopes, even if the far wings of their absorption lines overlap [107]. The combination of resonant multiphoton ionization (REMPI) with mass spectrometry for the investigation of molecular dynamics and fragmentation is discussed in Chap. 5. 

Fig. 10.3 Resonant multiphoton ionization (REMPI) as a sensitive detection technique for small quantities of atoms or molecules evaporated from a heated surface...

S. Fredin, D. Gauyacq, M. Horani, C. Jungen, G. Lefevre, F. Masnou-Seeuws, S and d Rydberg series of NO probed by double resonance multiphoton ionization. Mol. Phys. 60, 825 (1987) ... 

Becker CH and Gillen KT (1984) Surface analysis by non-resonant multiphoton ionization of desorbed or sputtered species. Analytical Chemistry 56 1671-1677. 

Kaesdorf S, Hartmann M, Schroder H, and Kompa KL (1992) Influence of laser parameters on the detection efficiency of sputtered neutrals mass spectrometry based on non-resonant multiphoton ionization. International Journal of Mass Spectrometry and Ion Processes 116 219-247. 

In the last paragraph of this report some brief comments are made concerning the usefulness of resonant multiphoton ionization for the construction of an cold ion beam source and the diagnostic use of photo-ionization in collisional energy transfer studies /4/. 

Muhlberger, F., Zimmermann, R., Kettrup, A. (2001) A Mobile Mass Spectrometer for Comprehensive On-line Analysis of Trace and Bulk Components of Complex Gas Mixtures Parallel Application of the Laser-based Ionization Methods VUV Single-photon Ionization, Resonant Multiphoton Ionization, and Laser-induced Electron Impact Ionization. Anal. Chem. 73 3590-3604. 

The combination of resonant laser excitation to an intermediate level and the subsequent mass analysis of the ion also makes the technique species selective. Therefore, one can distinguish REMPI spectra of systems that have the same mass, e.g. phenol-H2 and phenol-CO. On the other hand, care needs to be taken not to approach the REMPI experiment in a brute-force approach, as one may be tempted to increase weak signals by increasing the laser pulse energies. Non-resonant multiphoton ionization (MPI) processes may destroy the carefully adjusted species selectivity, as will be shown in the example of the REMPI investigation of CaH/CaD reaction products see Figure 9.5 below. 

The first experiments were carried out in 1983 [13.99,13.100]. The H atoms were produced by photodissociation of HJ molecules in an effusive beam using the fourth harmonics of Nd YAG lasers. Since the dissociated iodine atom is found in the two fine-structure levels /(P1/2) and /(P3/2), two groups of H atoms with translational energies kin = 0.55 eV or 1.3 eV in the center-of-mass system H-f-D2 are produced. If the slower H atoms collide with D2 they can reach vibrational-rotational excitation energies in the product molecule up to (u = 1, / = 3), while the faster group of H atoms can populate levels of HD up to (v = 3, J = 8). The internal-state distribution of the HD molecules can be monitored either by CARS (Sect. 8.3) or by resonant multiphoton ionization [13.99]. Because of their fundamental importance, these measurements have been repeated by several groups with other spectroscopic techniques that have improved signal-to-noise ratios [13.101]. 

The combination of REsonant MultiPhoton Ionization (REMPI) with mass spectrometry for the investigation of molecular dynamics and fragmentation is discussed in Chap. 10. 

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