Double resonance excitation

A typical overtone pumping experiment, with double-resonance excitation, is illustrated in the left-hand panel of Fig. 10 [126]. It involves three different laser pulses of 5-8 ns duration. The first laser promotes molecules to a specific vibrational-rotational intermediate state with two quanta in the OH stretch vibration. After a delay time the second pulse promotes only the preselected molecules to a higher OH stretch overtone level, that lies in the continuum and dissociates to OH and Cl. Finally, the OH fragments are detected by LIF using a third laser. 

The examples used here to illustrate the effects of a moderately strong magnetic field (Bz 1 Tesla) are from an experimental study of the NO nf <— A2E+ transition probed by double resonance excitation via a selected rotational-Zeeman sublevel of the A2E+(w = 1) state. Individual nf(N+)... 

The nonradiative decay pathways for the (X2E+ )np Rydberg states of H2 have been examined by McCormack, et al, (1993). Selected vibrational-rotational levels of the E, FXE+ state were excited by two-photon transitions from the X E+ state and the ungerade Rydberg states were examined by a double resonance excitation scheme via the selected levels of the E, F state. 

Figure 4.3 Energy level diagram for infrared-optical double resonance excitation of HOOH to the 6pqh vibrational overtone level and subsequent laser-induced fluorescence detection of the OH dissociation products. The 6vqh level is at 18,943 cm above the ground state. Since the O—O bond dissociation energy of HOOH is 17,035 cm molecules dissociating from 6vqh have 1913 cm of excess vibrational energy to be partitioned between the two OH fragments Luo and Rizzo, 1990).

Sequential double resonance excitation requires that both transitions fall within the range of suitable laser sources. However, it is often desirable to use population labelling to assign a single band system where this may not be possible. Figure 13b uses competitive excitation to seek pairs of transitions with a level in common. In this case it is necessary to partially saturate the pump transition in order that it may affect the fluorescence intensity of excitation via the probe laser. With an extended band system and c.w. lasers the pump beam may be chopped, with modulation detected on the fluorescence excited by the unchopped probe, a filter having been used to block the fluorescence from the pump. 

La (657.85), three-step RIMS, DL - first step Sr (689.45, 688.02), double-resonance excitation by two DLs... 

Fig. 6. Dispersed emission spectrum following double resonance excitation of NelCl to the vjq = 2 level.

Optical double-resonance excited by modulated light. 

Callegari A, Rebstein J, Muenter J S, Jost R and Rizzo T R 1999 The spectroscopy and intramolecular vibrational energy redistribution dynamics of HOCI in the u(OH) = 6 region, probed by infrared-visible double resonance overtone excitation J. Chem. Phys. 111 123-33... 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

Fig. 1 (Left) Schematic of frequency degenerate 2PA (1) into the first allowed singlet state, (2) above the first allowed singlet state, and (3) a double resonant condition, with a small intermediate state resonance energy difference, A, and a transition into an allowed final 2PA state. (Right) Photograph illustrating the much sharper contrast of two-photon (b) versus one-photon excitation (a) (taken from [2])...

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

A major limitation of CW double resonance methods is the sensitivity of the intensities of the transitions to the relative rates of spin relaxation processes. For that reason the peak intensities often convey little quantitative information about the numbers of spins involved and, in extreme cases, may be undetectable. This limitation can be especially severe for liquid samples where several relaxation pathways may have about the same rates. The situation is somewhat better in solids, especially at low temperatures, where some pathways are effectively frozen out. Fortunately, fewer limitations occur when pulsed radio and microwave fields are employed. In that case one can better adapt the excitation and detection timing to the rates of relaxation that are intrinsic to the sample.50 There are now several versions of pulsed ENDOR and other double resonance methods. Some of these methods also make it possible to separate in the time domain overlapping transitions that have different relaxation behavior, thereby improving the resolution of the spectrum. 

Chew and Wang(39) have pointed out the possibility of double resonance, that is, that the frequencies of both the excitation and inelastically scattered radiation are resonant. They presented the results of calculations which indicate that double resonance can have a significant effect on the angular intensity distribution of inelastically scattered radiation. This case is of some practical interest, particularly in Raman studies, where coincidence may lead to anomalous Raman band intensities, if both the excitation and the shifted frequency are resonant. 

The new techniques of phosphorescence-microwave multiplet resonance spectroscopy with optical detection have been reviewed by El-Sayed and Kwiram Such exciting experiments as the optical detection on electron-nuclear double resonance (ENDOR) and of electron-electron double resonance (EEDOR) in zero magnetic field have been achieved, and it is certain that much detailed knowledge concerning the phosphorescent states will evolve from this field. 

S2 - Sq fluorescence and radiationless transitions from the state of porphyrins have been studied in order to reveal photodynaunics of porphyrins. The S2 state fluorescence of zinc(II)-tetraphenylporphin is caused even by the excitation to the state. Two-photon absorption and optical-optical double resonance studies show that a stepwise two-photon absorption through the state is a main process populating the S2 state. 

VMP, a term coined by Crim and coworkers in 1987 [26], has been applied extensively by him and others since then [27-33], However, some VMP studies were carried out even earlier (see Section 2.3). As mentioned earlier, direct photodissociation, Figure 2.1a, involves electronic excitation of vibrationless ground state parent molecules to an excited PES, while in VMP, Figure 2.1b, excitation of molecules initially prepared in a particular vibrational state is involved. Therefore, in VMP, two- or three-color photons are required to induce a double resonant... 

---