Absorption-depletion spectroscopy

This section will present two selected examples of electronic spectroscopy on mass-selected metal clusters in the gas phase. In the first example, time-resolved photoelectron spectroscopy is employed to monitor the real time evolution of an electronic excitation leading to the thermal desorption of an adsorbate molecule from a small gold cluster. In the second example, optical absorption-depletion spectroscopy in conjunction with first principles calculations provide insight into the excited state structure of mass-selected metal clusters. 

Absorption spectroscopy records depletion by the sample of radiant energy from a continuous or frequency-tunable source, at resonance frequencies that are characteristic of various energy levels ia atoms or molecules. The basic law of absorption, credited to Bouguer-Lambert-Beer, states that ia terms of the iacident, Jq, and transmitted, light iatensities, the absorbance, M (or transmittance, T), is given by equation 1 ... 

X-ray absorption spectroscopy has been performed on the isolated Rieske protein from bovine heart mitochondrial bc complex 69) as well as on the Rieske-type cluster in Burkholderia cepacia phthalate dioxygenase (PDO) (72). The analysis performed by Powers et al. 69) was significantly hampered by the fact that the presence of two histidine ligands was not fully recognized therefore, only the results obtained with the dioxygenase where the mononuclear iron has been depleted will be considered here. Table VII gives a comparison of the distances obtained from the fit of the EXAFS spectra assuming an idealized Rieske model and of the distances in the crystal structures... 

Vj = 1 <— v" = 1 transition will be at a different energy than the Vj = 0 <— v" = 0. We use this fact to measure the vibrational spectrum of V (OCO) in a depletion experiment (Fig. 12a). A visible laser is set to the Vj = 0 Vj = 0 transition at 15,801 cm producing fragment ions. A tunable IR laser fires before the visible laser. Absorption of IR photons removes population from the ground state, which is observed as a decrease in the fragment ion signal. This technique is a variation of ion-dip spectroscopy, in which ions produced by 1 + 1 REMPI are monitored as an IR laser is tuned. Ion-dip spectroscopy has been used by several groups to study vibrations of neutral clusters and biomolecules [157-162]. 

The presence of calcium in horseradish peroxidase was demonstrated originally by Haschke and Friedhoff, working with the C and A (imspec-ified, but likely to have been predominantly A2) isoenzymes (209). HRP C and HRP A contain 2.0 0.13 and 1.4 0.19 moles calcium per mole enzyme, respectively, as determined by atomic absorption spectroscopy. Incubation in 6 M guanidinium hydrochloride and 10 mM EDTA for 4 hours at neutral pH and room temperature gave calcium-depleted enzymes with specific activities decreased by 40% and 15%, respectively. The thermal stability of calcium-depleted HRP C was also reduced compared to native enzyme. Reconstitution was successful only with calcium-depleted HRP C (209). It remains to be established whether this reflects true structural differences between the calcium binding sites of the two isoenz5unes, or is a consequence of the relatively harsh... 

High pressure infrared (HP IR) spectroscopy has now been used for over 30 years for the study of homogeneous transition metal catalysed processes. The technique is particularly useful for reactions involving carbon monoxide, for which transition metal carbonyl complexes are key intermediates in the catalytic mechanisms. Such complexes have one or more strong r(CO) absorptions, the frequencies and relative intensities of which provide information about the geometry and electronic character of the metal center. As well as probing the metal species, HP IR spectroscopy can also be used to monitor the depletion and formation of organic reactants and products if they have appropriate IR absorptions. 

Fig. 3. Pump-deplete-probe spectroscopy on lycopene in hexane, a) Experimental setup After excitation and depletion of Car S2 with a delay of r=50fs, a white-light probe pulse at delay tprob<.=2ps measures the transient absorption spectrum, b) Spectra without (solid curve) and with depletion pulse (dotted) and their difference (shaded area). Only the Car Si state is depleted the ground state bleach (S0-S2) and positive absorption feature on its low energy side (hotSo-S2) are unaffected.

The experimental configuration of the pump-probe experiment is similar to Ref. [5]. A home built non-collinear optical parametric amplifier (nc-OPA) was used as a pump, providing Fourier-transform-limited 30 fs pulses, which could be spectrally tuned between 480-560 nm. In all experiments white-light generated in a sapphire crystal using part of the fundamental laser (800 nm), was used as probe light. In the pump-probe experiments the pump was tuned to the S2 0-0 band for carotenoids with n>l 1. In the case of M9, it was not possible to tune the nc-OPA to its 0-0 transition, and hence another nc-OPA tuned to 900 nm was frequency doubled and used for excitation. In addition to conventional transient absorption pump-probe measurements, we introduce pump-deplete-probe spectroscopy, which is sensitive to the function of an absorbing state within the deactivation network. In this technique, we... 

After a short period of use in the average engine, changes start to occur. Initially, a loss of the zinc based antiwear/antioxidant additive ZDDP is observed by negative absorptions at 1000 cm 1 and 715 cm 1. Oxidative degradation of oil follows soon after and this is observed by positive absorptions, represented by carbonyl, hydroxy, nitro and C-O- species. The ER spectroscopy of lubricants can reflect additive depletion and the formation of oxidation products (Coates and Setti, 1984 Coates etal., 1984). 

Figure 50. Snapshots of oxygen incorporation experiments in Fe-doped SrTi03, recorded by in situ time and space resolved optical absorption spectroscopy.256 Rhs column refers to the corresponding oxygen concentration profiles, in a normalized representation. Top row refers a predominantly diffusion controlled case (single crystal), center row to a predominandy surface reaction controlled case (single crystal), bottom row to transport across depletion layers at a bicrystal interface.257,258 For more details on temperature, partial pressure, doping content, structure see Part I and Ref.257-259 Reprinted from J. Maier, Solid State Ionics, 135 (2000) 575-588. Copyright 2000 with permission from Elsevier.

Two-color pump-probe absorption spectroscopy is carried out with moderate pump energies producing small depletions of the vibrational ground state of only a few percent in order to avoid secondary excitation steps and minimize the temperature increase of the sample due to the deposited pump energy. 

Huppert et al. have recently applied picosecond and nanosecond flash spectroscopy to methanol solutions of 11-cis PRSB (196). Monitoring at 485 nm which is within the main absorption band of the molecule, they observe a fast (<10 ps) depletion, followed by a slow (tr 11 ns) recovery of the absorbance to a final permanent value consistent with a net (11-cis) -+ (all-trans) interconversion. In the red, between 550 and 660 nm, they observe a fast rise in absorbance (<10 ps) due to an unidentified long-lived (Td 0.5 ns) transient (X). Although unclear as to whether Tr = Td they suggested that X is an (excited-state or ground-state) isomer precursor of the final all-trans photoproduct. It was thus concluded that photoisomerization is a relatively slow process occurring in the nanosecond range. Independent of the identity of X, this conclusion does not seem to be a unique interpretation of the experimental observations noted above. In fact, these are consistent with either one of two mechanisms ... 

All these studies with femtosecond pulses on the primary photochemical processes of rhodopsin were done by means of transient absorption (pump probe) spectroscopy [10]. However, absorption spectroscopy may not be the best way to probe the excited-state dynamics of rhodopsin, because other spectral features, such as ground-state depletion and product absorption, are possibly superimposed on the excited-state spectral features (absorption and stimulated emission) in the obtained data. Each spectral feature may even vary in the femtosecond time domain, which provides further difficulty in analyzing the data. In contrast, fluorescence spectroscopy focuses only on the excited-state processes, so that the excited-state dynamics can be observed more directly. 

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