Double photon ionization

Figure 10 Double photon ionization of liquid tetramethylsilane. — 249 nm — 308 nm 0 — 353 nm. (Redrawn from the data of Bottcher, E. H. and Schmidt, W. F., Z. Naturforsch, 36a, 406, 1981b.)...

Two-Photon Ionization of 2-Aminopurine in Single- and Double-Stranded DNA... 

Transient spectra of solvated indole are measured in a 120 Jim liquid jet with a crosscorrelation of 80 fs by means of a white light continuum (450 - 740 nm) generated in a sapphire disc. The molecules are excited at 270 nm with pump pulses generated by frequency doubling the output of a noncollinearly phase matched optical parametric amplifier [2], Due to the short pump pulses there is a small yet finite probability for two-photon ionization in pure solvents. This allows us to study the spectral properties of the generated solvated electrons by measurements in pure solvents. The transient spectra of the indole solution are corrected for these solvent contributions. 

Figure 12-1. Schematic diagram to illustrate double resonance techniques, (a) REMPI 2 photon ionization. The REMPI wavelength is scanned, while a specific ion mass is monitored to obtain a mass dependent SI <- SO excitation spectrum, (b) UV-UV double resonance. One UV laser is scanned and serves as a burn laser, while a second REMPI pulse is fired with a delay of about 100 ns and serves as a probe . The probe wavelength is fixed at the resonance of specific isomer. When the burn laser is tuned to a resonance of the same isomer it depletes the ground state which is recorded as a decrease (or ion dip) in the ion signal from the probe laser, (c) IR-UV double resonance spectroscopy, in which the burn laser is an IR laser. The ion-dip spectrum reflects the ground state IR transitions of the specific isomer that is probed by the REMPI laser, (d) Double resonance spectroscopy can also use laser induced fluorescence as the probe, however that arrangement lacks the mass selection afforded by the REMPI probe...

Figure 12-2 shows data we obtained for three isolated GC base pair structures. Row A shows results for the Watson-Crick (WC) structure, while rows B and C represent the second and third lowest energy structures, respectively, which are not WC. The second column shows the IR-UV double resonance data, compared with the ab initio calculations of the vibrational frequencies. These data allow us to assign the structures. The third column shows the UV excitation spectra, measured by resonant two-photon ionization (R2PI). The UV spectrum is broad for the WC structure (A) and exhibits sharp vibronic lines for the other structures. 

Infrared-UV double resonance has proved to be very useful for the solution of the conformational structure of large bio-molecules [621]. Here the absorption of an infrared photon on a selected vibrational transition is detected by two-photon ionization of the excited vibrational level. In Fig. 5.44 the infrared UV-double resonance spectra for the two lowest energy conformations of the MAPE molecule (2-methylamino 1-phenyl ethanol) [621]. The lower level of the 4 IR-transitions A B C and D can be labelled by tuning the IR-laser on the corresponding transitions which depletes the lower levels and weakens the selected line in the absorption spectrum. 

Figure 7.8. Classification of various resonance ionization schemes for the ionization of atomic species, ooi and 002 represent photons of frequency 1 and frequency 2, respectively, and 2 coi, frequency-doubled photons. (Reproduced from ref. 25 by permission of the American Physical Society, College Park, MD, copyright 1979.)...

Resonance enhanced two-photon ionization via the A <— X y(O-O) and v(l-l) bands was used for state-specific detection of NO. Frequency doubling the output of a XeCl excimer pumped dye laser in a potassium pentaborate crystal produced tunable ultraviolet radiation for the ionization with UV pulse energies of approximately 30 microjoules in a bandwidth of about 0.4 cm. The focussed UV beam crossed the molecular beam at right angles and could be moved over a variety of radii and angles about the scattering sample surface. Ionized NO molecules were detected using a Johnston MM-1 multiplier. 

G. Delacretaz and L. Woste, Two-Photon Ionization Spectroscopy of the (2y El Double-Minimum State of Na2 , Chem. Phys. Lett. 120, 342 (1985). 

More essential is the dynamical polarization of the 4d. shell by the incoming photon. Therefore the self-consistent field acting upon outer electrons becomes time-dependent, and easily ionizes them. As a result the single charge ion yield has a powerful maximum of collective nature in the 4d-threshold region. The double electron ionization cross section also increases, even below the 4d-threshold, which may be explained by dynamical polarization of the 4d subshell. ... 

When the states P1 and P2 are described as linear combinations of CSFs as introduced earlier ( Fi = Zk CiKK), these matrix elements can be expressed in terms of CSF-based matrix elements < K I eri IOl >. The fact that the electric dipole operator is a one-electron operator, in combination with the SC rules, guarantees that only states for which the dominant determinants differ by at most a single spin-orbital (i.e., those which are "singly excited") can be connected via electric dipole transitions through first order (i.e., in a one-photon transition to which the < Fi Ii eri F2 > matrix elements pertain). It is for this reason that light with energy adequate to ionize or excite deep core electrons in atoms or molecules usually causes such ionization or excitation rather than double ionization or excitation of valence-level electrons the latter are two-electron events. 

A different strategy has been applied in our work, that emphasizes the importance of DNA stability on hole transfer within double-stranded DNA. This work is based on determination of the overall yield of oxidized nucleosides that arise from the conversion of initially generated purine and pyrimidine radical cations within DNA exposed to two-photon UVC laser pulses. On the one hand, this work benefits from the excellent current knowledge of chemical reactions involving the radical cations of DNA bases, and on the other hand, from major analytical improvements that include recent availability of the powerful technique of high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (CLHP-ESI-MS/MS) [16-18]. 

Figure 2 The photoabsorption (c), photoionization (o-,-), and photodissociation (cr

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