Multiple-photon excitation

Wokosin D L and White J G 1997 Optimization of the design of a multiple-photon excitation laser scanning fluorescence imaging system Proc. SPIE 2984 25-9... 

V. S. Letokhov, Laser Nonlinear Chemistry with Multiple Photon Excitation, Springer-Verlag, Berlin, 1983. 

A typical problem of interest at Los Alamos is the solution of the infrared multiple photon excitation dynamics of sulfur hexafluoride. This very problem has been quite popular in the literature in the past few years. (7) The solution of this problem is modeled by a molecular Hamiltonian which explicitly treats the asymmetric stretch ladder of the molecule coupled implicitly to the other molecular degrees of freedom. (See Fig. 12.) We consider the the first seven vibrational states of the mode of SF (6v ) the octahedral symmetry of the SF molecule makes these vibrational levels degenerate, and coupling between vibrational and rotational motion splits these degeneracies slightly. Furthermore, there is a rotational manifold of states associated with each vibrational level. Even to describe the zeroth-order level states of this molecule is itself a fairly complicated problem. Now if we were to include collisions in our model of multiple photon excitation of SF, e wou d have to solve a matrix Bloch equation with a minimum of 84 x 84 elements. Clearly such a problem is beyond our current abilities, so in fact we neglect collisional effects in order to stay with a Schrodinger picture of the excitation dynamics. 

Figure 12. Schematic of multiple photon excitation dynamics of SFe. Groups of levels show lowest three vs vibrational states. Higher states are split by rotational interactions with vibrational motion.

There are several other interesting topics in quantum optics which we would like to be able to study. For example, we would like model problems in double resonance spectroscopy, where there are two electromagnetic fields with possibly different polarizations simultaneously interacting with a molecule. This problem resembles the multiple photon excitation problem in that there is population migration along ladders of states, but in this case there can be a vastly larger number of quantum levels to treat — on the order of 2(2J+1). At room temperature, the most probable value of J for SF is about 60, which implies a 250 state calculation. 

This Section is subdivided into three parts the first deals with studies of multiple-photon excitation and the second deals with molecular decomposition induced by i.r. radiation. The third Section covers briefly aspects of isotope separation. 

Multiple-photon Excitation.—For researchers interested in understanding the behaviour of polyatomic molecules subjected to intense i.r. laser radiation the molecule SF has become a laboratory standard . Consequently, there have been many studies of the IRMPE of SFe of both an experimental and a theoretical nature. These are summarized in Table 10. 

A study has appeared which shows that U02 bound to colloidal silica has significantly different photophysical properties from those of the free ion in aqueous solution. The luminescence intensity and lifetime of U02 in H2SO4 and H3PO4 have been determined in the presence of Np, Np, and Np ". Of these, Np was found to be the most effective quencher. Real-time detection of multiple photon excitation of the ground state of U02(hfacac)2TMP (hfacac = hexafluroacetylacetonato) has been described. 

Figure 10. Survival fraction of iron-methanol complexes Fe,(CDjOH) upon infrared multiple-photon excitation as a function of CO2 laser fluence at 956.2 cm. Notation n, m refers to number of iron atoms and methanol molecules, respectively, in complex. (From Zakin et al. )...

M. N. R. Ashfold and G. Hancock, Infrared Multiple Photon Excitation and Dissociation Reaction Kinetics and Radical Formation , in Gas Kinetics and Energy Transfer, ed. P. G. Ashmore and R. J. Donovan (Specialist Periodical Reports), The Royal Society of Chemistry, London, 1981, Vol. 4, p. 73. 

C.D. Cantrell (ed.). Multiple-Photon Excitation and Dissociation of Polyatomic Molecules. Springer Topics. Curr. Phys., vol. 35 (Springer, Berlin, 1986) ... 

As the laser intensity is increased, the probability that a molecule will absorb more than one photon increases rapidly. When the initial excitation is to a real intermediate state, further excitation is efficient and can lead directly to ionization, if the energy of the photons is sufficient this forms the basis of the simplest REMPI scheme (i.e. 1 + 1 REMPI) described in Chapter 9. Excitation processes of this type are referred to as multiple-photon excitation, as the two steps are sequential and essentially independent (i.e. the process is not coherent). If, in the second step, another tuneable laser is used to further excite the molecule, from the real intermediate state, threshold ionization processes can be studied, and this is exploited in the ZEKE technique, which provides high-resolution photoelectron spectra of molecules (see Section 18.3). 

The EOg state of I2 then undergoes fluorescence, producing an oscillatory continuum, which leads to dissociation (see Chapter 15.5). An interesting consequence of multiple-photon excitation is that each step partially and successively aligns the population of the excited states, an extension of the process shown in Figure 15.3. 

Repulsive (real) intermediate states have also been used for multiple-photon excitation, despite their short lifetime. This approach allows the Franck-Condon window to be extended significantly and it has been used to study the Rydberg and ion-pair states of a number of molecules, including diatomic halogens and methyl iodide (see Section 18.2). 

Figure 18.1 Schematic representation oftheIR multiple-photon excitation of a polyatomic molecule (exemplified for SFg). The excitation process can be divided into three parts (i) the first three absorption steps require rotational compensation in order to achieve resonance with the fixed-frequency laser (see text) (ii)afterthat, excitation through the quasi-continuum ensures resonance and further absorption up to the dissociation threshold (iii) dissociation starts at threshold and the dissociation rate increases rapidly with excess energy...

However, it is important to recognize that the first few steps in the absorption process are selective. Owing to the low density of states in this region, only one molecular species (which has a transition resonant with the laser frequency) in a mixmre of other molecules will absorb and thus be selectively excited into the quasicontinuum and on to the dissociation limit. Indeed, it is possible to achieve isotope separation using IR multiple-photon excitation for example, can be selec-... 

Because of the low collision rate in the high vacuum environment of a Fourier transform mass spectrometer (FTMS), vibrationally excited molecular ions cool predominantly by IR fluorescence. For typical IR transition dipole moments and frequencies in the mid-IR, spontaneous emission is expected to occur at a rate in the range of 1-100 s To energize an ion efficiently using IR multiple-photon excitation (MPE), the rate of photon absorption - the product of absorption cross section and photon flux - should exceed the emission rate. From such a back-of-an-envelope estimate, one finds that radiation sources producing several Watts/cm are required to induce efficient dissociation [141], Note that the demands on laser power may further increase because of the limited residence time of the ions in the laser field, collisional deactivation in traps at higher pressures, limited spectral overlap between molecular absorption and laser emission profiles, etc. 

As a consequence of the IVR-mediated nature of the multiple-photon excitation process, the vibrational excitation is randomized as the dissociatirai threshold is approached. Hence, the molecule has no memory of the vibrational coordinate that was originally excited. Dissociation therefore occims statistically and can be modelled using the Arrhenius equation or phase-space theories. Mode-selective dissociation is normally not observed. 

Basics of IR multiple-photon excitation/dissociation of polyatomic molecules in the ground state... 

Multiple-photon excitation of lower vibrational levels... 

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