Two Colour Excitation

The irradiation of atoms and molecules with two lasers of different frequency and known relative phase has been shown not only to throw light on the ATI process in atoms [46] but has produced a dramatic enhancement in the conversion efficiency in high harmonic generation [47]. In addition, there is theoretical evidence that molecular dynamics may be controlled using two-colour excitation [48]. 

In the present chapter, the rapidly growing subject of atoms in strong laser fields has been briefly described, the main emphasis being on the novel effects which have been observed. Several aspects of the problem have not been discussed for example, above 1020 Wcm-2, relativistic effects will become important, although these have not yet been observed. Similarly, we have omitted any discussion of coherent control in two-colour excitation and femtosecond chemistry. 

Figure 18.2 Schematic diagram showing the potential curves for the ground state, the repulsive intermediate states and two of the Rydberg states of CH3I. The vertical arrows show the one-colour, non-resonant (dashed arrows), and the two-colour, resonant (solid arrows) routes for two-photon excitation to the Rydberg states. Note that resonance with the repulsive intermediate state (two-colour excitation) leads to stretching of the C-I bond and this changes the Franck-Condon window for excitation to the Rydberg state, favouring the C-I vibrational mode V3. Reproduced from Min etal, J. Photochem. Photobiol., 1996, 100 9, with permission of Elsevier...

Figure 3 A two-colour fluorescence depletion spectrum of one rovibronic line associated with the D n <- A n transition in SO. The two-colour excitation scheme used (upper right) is required because of the very short lifetime (100 fs) of the D P state. This results in the linewidth of 50 cm- shown in the spectrum.

In this paper we report the use of spectrally resolved two-colour three-pulse photon echoes to expand the information that can be obtained from time-resolved vibrational spectroscopy. The experiments allow the study of intramolecular dynamics and vibrational structure in both the ground and excited electronic states and demonstrate the potential of the technique for studying structural dynamics. 

A. E. Kondo, W. J. Meath, S. H. Nilar, and A. J. Thakkar, Chem. Phys., 186, 375 (1994). Pump-Probe Studies of the Effects of Permanent Dipoles in One- and Two-Colour Molecular Excitations. 

Two-colour photoionization spectroscopy of aniline cooled in a supersonic jet. Strong propensity for vertical (An = 0) ionization allows vibrational frequencies of CgHgNH2 ( B,) to be determined Two-colour photoacoustic and MPI spectra of aniline, determined as a function of time delay between the two laser pulses. Observed both ionization and dissociation t MPI/TOF mass spectrometric study of phenol. Mechanism of ionization and ion fragmentation t MPI/TOF mass spectrometric study of fragmentation patterns in benzaldehyde. Strong wavelength dependence observed at 266 and 355 nm. Results show operation of two different mechanisms at these excitation wavelengths... 

In connection with asymmetry, an interesting experimental point arises its influence on the two-dimensional graphs can in principle be probed directly in two-colour laser experiments, in which the same excited Rydberg... 

As regards the high laser field problem, extensions include the so-called two-colour experiments [589] in which coherent mixtures of two laser fields of different frequency are used to tailor the excitation. The present chapter merely provides an introduction to research on atoms in strong fields, both oscillatory and constant in time, which are rapidly developing areas of atomic and molecular physics. 

In sections 4.2 and 4.4 emission spectroscopy provided more information than could be obtained from an excitation spectrum. Even so, the weakness of such emission makes it difficult to obtain high resolution in its dispersed spectrum, and data are only obtained about levels lying below that excited. Two-colour double resonance can remove both these limitations in favourable cases. We may envisage three basic schemes for optical-optical double resonance using two lasers. 

Fig. 13 Level schemes for two-colour optical-optical double resonance Either the pump laser or the probe laser may be scanned with the frequency of the other laser held fixed, (a) Sequential OODR. (b) Competitive excitation. (c) Stimulated emission pumping.

In parallel with these experiments, the adiabatic PESs for the ground, ionic and excited states, together with non-adiabatic couplings, were calculated ab initio. Wave-packet dynamics simulations were then carried out on these smfaces and a detailed mechanism was deduced by comparing the results with the experiments using adaptive optimal control and some further two-colour femtosecond pump-probe experiments. 

In recent years, there have been several successful attempts to use lasers for re-exciting ions after they have been cooled down. Since the ion cloud consists of a limited number, various strategies are possible for deriving information on state specific collision dynamics. The methods range from a dedicated perturbation of a stationary low temperature equilibrium via burning a hole into the state population to two colour pump — probe experiments. Most of such activities are used for spectroscopy or for understanding the low temperature populations of trapped hydrogen ions. They are discussed in Chapter 6. 

DsRed and its mutants display emission spectra sufficiently red-shifted for two-colour applications with GFP mutants. The only combination of GFP mutants to display a similar small spectral overlap is presented by EBFP and EYFP, but no FCS application of EBFP has been reported so far and its excitation max-imiun (388 nm) is unfavourable for intracellular applications. 

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