Resonant two-photon excitation

For the case of resonance, two-photon excitation into the two-photon al-... 

To our knowledge, 46 has never been observed in solution under stable conditions, even at low temperature. Pulse radiolysis " of benzyl chloride as well as flash photolysis ° of several derivatives in HHP have allowed the observation of the electronic absorption spectra of benzyl and its 4-methyl and 4-methoxy derivatives. The and NMR spectra of the 2,4,6-trimethylbenzyl cation and other more heavily substituted benzyl cations, however, have been studied at low temperature in superacid media. In the gas phase, cold benzyl radical has been probed by two-color, resonant two-photon ionization techniques, thus providing very accurate vibrational frequencies below 650 cm for the benzyl cation. Furthermore, the adiabatic ionization energy of benzyl radical and several isotopomers in the ground state were determined from their threshold photoionization spectra using resonant two-photon excitation and detection of electrons by pulsed field ionization. This information, combined with Af//° (CgH5CH2) from Ref. 212 leads to the value of Af//°m(46) reported in Table 9. 

Another alternative is the resonant two-photon excitation of a high lying level which is then ionized by a third photon from the same or from another laser (Fig. 1.36d). With resonant three-photon excitation excited states with opposite parity are reached which are then ionized by a fourth photon (Fig. 1.36e). The smallest ionization probability has the non-resonant two-photon ionization (Fig. 1.36f), which can still yield enough sensitivity if pulsed lasers with sufficient output power are used. 

In contrast to nonresonant two-photon excitation, the cross-section for resonant two-photon excitations is relative large if the atoms are excited via a strong resonance transition. To achieve resonant two-photon excitation, however, two tunable lasers are necessary with sufficiently narrow spectral bandwidths. The laser beams intersect the absorbing volume in CO- or counter-propagating direction. If the first laser is tuned to the Doppler profile of the lower transition, atoms are excited with a well-defined velocity component in beam direction, whereas the second laser probes the population density of the excited atoms within this velocity group (Figure 4). The basic arrangement for isotope-selective analysis makes use of two absorption volumes, which are intersected by... 

Figure 4 Principle of Doppler-free two-photon excitation (A) resonant two-photon excitation steps (B) frequencies of coun-terpropagating laser beams, as seen by atoms of the velocity component v (C) population densities of competing levels according to a well-defined class of velocity of analyte atoms (D) resulting Doppler-free spectrum.

We carried out experiments on resonant two photon excitation of high... 

More commonly, the resonant two-photon process in Figure 9.50(c) is employed. This necessitates the use of two lasers, one at a fixed wavenumber Vj and the other at a wavenumber V2 which is tunable. The first photon takes the molecule, which, again, is usually in a supersonic jet, to the zero-point vibrational level of an excited electronic state M. The wavenumber of the second photon is tuned across the M to band system while, in principle, the photoelectrons with zero kinetic energy are detected. In practice, however, this technique cannot easily distinguish between electrons which have zero kinetic energy (zero velocity) and those having almost zero kinetic energy, say about 0.1 meV... 

Figure 9.51 A zero kinetic energy photoelectron (ZEKE-PE) resonant two-photon spectrum of 1,4-difluorobenzene in which the first photon excites the molecule of the zero-point level of the S-[ excited electronic state of the molecule. (Reproduced, with permission, from Reiser, G., Rieger, D., Wright, T.G., Muller-Dethlefs, K. and Schlag, E.W., J. Phys. Chem., 97, 4335, 1993)...

LaMorte, Y. J., Zoumi, A. and Tromberg, B. J. (2003). Spectroscopic approach for monitoring two-photon excited fluorescence resonance energy transfer from homodimers at the subcellular level. J. Biomed. Opt. 8, 357-61. 

Clapp AR et al (2007) Two-Photon excitation of quantum-dot-based fluorescence resonance energy transfer and its applications. Adv Mater 19 1921-1926... 

The simplest version of REMPI uses a two photon (1-1-1) process, namely resonant two photon ionization (R2PI). In this, the species M is first promoted from its electronic ground state So to the electronic excited state Si via a resonant absorption step. Then, the non-resonant absorption of a second photon takes the species into the ionization continuum. If the frequencies of the excitation and ionization photons are equal, the process is named one color R2PI (lcR2PI), otherwise two colors R2PI (2cR2PI) (Fig. 1). 

In this chapter we explore several aspects of interferometric nonlinear microscopy. Our discussion is limited to methods that employ narrowband laser excitation i.e., interferences in the spectral domain are beyond the scope of this chapter. Phase-controlled spectral interferometry has been used extensively in broadband CARS microspectroscopy (Cui et al. 2006 Dudovich et al. 2002 Kee et al. 2006 Lim et al. 2005 Marks and Boppart 2004 Oron et al. 2003 Vacano et al. 2006), in addition to several applications in SHG (Tang et al. 2006) and two-photon excited fluorescence microscopy (Ando et al. 2002 Chuntonov et al. 2008 Dudovich et al. 2001 Tang et al. 2006). Here, we focus on interferences in the temporal and spatial domains for the purpose of generating new contrast mechanisms in the nonlinear imaging microscope. Special emphasis is given to the CARS technique, because it is sensitive to the phase response of the sample caused by the presence of spectroscopic resonances. 

TheM vs. N photon scenarios, where both routes are nonresonant, were discussed in Section 3.3. Here we consider resonantly enhanced routes and show, in particular,, jhat a resonantly enhanced two-photon vs. two-photon excitation (see Fig. 6.1) provides a means of maintaining control in a molecular system in thermal equilibrium, The resonant character of the excitations ensure that only a particular initial... 

Resonantly enhanced two-photon dissociation of Na2 from a bound state of the. ground electronic state occurs [202] by initial excitation to an excited intermediate bound state Em,Jm, Mm). The latter is a superposition of states of the A1 1+ and b3Il electronic curves, a consequence of spin-orbit coupling. The continuum states reached in the two-photon excitation can have either a singlet or a triplet character, but, despite the multitude of electronic states involved in the computation reported J below, the predominant contributions to the products Na(3s) + Na(3p) and Na(3s) + Na(4s) are found to come from the 1 flg and 3 + electronic states, respectively. The resonant character of the two-photon excitation allows tire selection of a Single initial state from a thermal ensemble here results for vt = Ji — 0, where vt,./, denote the vibrational and rotational quantum numbers of the initial state, are stJjseussed. 

At low density (< 1012 cm-3) and temperatures > 100 /jK the two-photon lineshape is a double exponential, exp(- p /<5p0) [3], as expected for Doppler-free two-photon excitation by a Gaussian laser beam of a thermal gas [29]. Here v is the laser detuning from resonance and 8v0 is the linewidth due to the finite interaction time of the atom with the laser beam. At low temperature, lines as narrow as 3 kHz (FWHM at 243 nm) have been observed. A detailed discussion of this lineshape in the trap and the appearance of sidebands due to coherence effects for repeated crossing of the laser beam can be found in [30]. 

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. 

---