State-resolved detection

As illustrated earlier in the text (Figure 10.5), molecules released from the centrifuge generate an oscillatory Raman signal, characteristic of the coherent rotation with well-defined relative phase relation between the quantum states inside a rotational wave packet. Time-resolved coherent Raman response from a wave packet centered at A = 69 in oxygen is plotted at the bottom of Figure 10.9a. Knowing the wave packet composition from the state-resolved detection discussed above. 

Laser state-to-state techniques include both the application of highly sensitive laser spectroscopy for internal state-resolved detection of molecules in the gas phase, e.g., desorbing or scattering from a surface, and second, for laser pumping an initial state prior to interaction with a surface. To date, laser detection of internal states has been widely applied in gas-surface dynamics experiments, while those involving optical state preparation techniques have only been applied in a limited fashion. 

Combining molecular beam techniques with laser state-resolved detection techniques has allowed state-resolved scattering measurements. 

Of, V = 0, 7 , Ef, 6f, vf, Jf, f Mf, Ts) for several systems. Tliis internal state-resolved detection in scattering is essential to understand how molecules make/break bonds at surfaces because of the strong scrambling of molecular modes in impact with the surface. Some of these experiments are discussed in Section 4.2. 

Fujii, R. et al., Cis-to-trans isomerization of spheroidene in the triplet state as detected by time-resolved absorption spectroscopy, J. Phys. Chem. A, 106, 2410, 2002. Montenegro, M.A. et al., Model studies on the photosensitized isomerization of bixin, J. Agric. Food Chem., 52, 367, 2004. 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

State-resolved inelastic scattering for a wide range of incident conditions ( ), d,) are measured for this system by combining molecular beam techniques with (2 + 1) ion TOF REMPI detection of the scattered molecules [58]. Energy transfer parallel to the surface is measured from the Doppler broadening of the REMPI spectra. Trapping... 

The most commonly used method is the direct measurement of a decay rate by pulsed excitation and time resolved detection. The most straightforward example of this technique is laser induced fluorescence applied to alkali Rydberg atoms. Alkali atoms are typically contained in a glass cell, which also holds a known pressure of perturber gas. The alkali atoms are excited to the Rydberg state at time t = 0 and the time resolved fluorescence from the Rydberg atoms is detected... 

Rizzo, T.R., Hayden, C.C., and Crim, F.F. (1984). State-resolved product detection in the overtone vibration initiated unimolecular decomposition of HOOH(6i oh) J- Chem. Phys. 81, 4501-4509. 

Antibodies coated onto MTP wells capture kinase or phosphatase substrate phosphorylation state is detected by anti-phosphopeptide antibody coupled to detector dye can be read by time-resolved fluorescence (DELFIA) technique... 

In conclusion, time-resolved excitation spectroscopy or, more correctly, excitation spectroscopy with time-resolved detection of emission, opens access to studies of intra- and inter-system crossing paths, i.e. of relaxation paths within or between hypersurfaces of different triplet substate systems. This method -applied for the first time in our investigation [60] for transition metal complexes - complements other measurements of pico-/subpico-second time resolution. In particular, it is shown that after an excitation of a vibrational state of an excited electronic triplet substate, the relaxation proceeds within the same triplet substate system downwards to the zero-point vibrational level. Subsequently, an inter-system crossing to a different sublevel system occurs in a relatively slow process by spin-lattice relaxation. This result fits well to the concept that a spin-flip is usually slower than the process of intra-state relaxation. 

By monitoring excitation spectra with a time-resolved detection of the emission, briefly called time-resolved excitation spectroscopy , it is possible, to identify specific relaxation paths. Although, these occur on a ps time scale, only measurements with a ps time resolution are required. It is shown that the relaxation from an excited vibrational state of an individual triplet sublevel takes place by a fast process of intra-system relaxation (on the order of 1 ps) within the same potential surface to its zero-point vibrational level. Only subsequently, a relatively slow crossing to a different sublevel is possible. This latter process is determined by the slow spin-lattice relaxation. A crossing at the energy of an excited vibrational/phonon level from this potential hypersurface to the one of a different substate does not occur (Fig. 24, Ref. [60]). This method of time-resolved excitation spectroscopy, applied for the first time to transition metal complexes, can also be utilized to resolve spectrally overlapping excited state vibrational satellites and to assign these to their triplet substates. 

Until quite recently, direct measurements of o(>d2)(X) were limited by the very real experimental difficulties associated with the highly efficient deactivation of O ( D2) by O3, as well as the need to provide a sensitive probe for atomic oxygen atoms in the ground Pj state as well as in the electronically excited D2 state. The development of resonance spectroscopic techniques for time-resolved detection of O ( Pi) has permitted monitoring of this state at densities of ca. 10 cm with an instrumental bandwidth in excess of 10 MHz. When combined with the use of high intensity photolysis sources such as the excimer lasers and frequency quadrupled Nd/YAG, it has proved possible to measure directly the yield of 0( D2) and O( Pj) at several discrete wavelengths in the middle ultraviolet. 

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