Modulator molecular ions

If cellular redox state, determined by the glutathione status of the heart, plays a role in the modulation of ion transporter activity in cardiac tissue, it is important to identify possible mechanisms by which these effects are mediated. Protein S-,thiolation is a process that was originally used to describe the formation of adducts of proteins with low molecular thiols such as glutathione (Miller etal., 1990). In view of the significant alterations of cardiac glutathione status (GSH and GSSG) and ion-transporter activity during oxidant stress, the process of S-thiolation may be responsible for modifications of protein structure and function. 

Sometimes one has to live with this problem, and some of the finest work, particularly on transient molecular species, has been achieved using source modulation. This includes some of the earliest work on neutral free radicals, by Kewley, Sastry, Winnewisser and Gordy [5], who studied the SO and CS species. With the benefit of hindsight, we know these radicals to be very long-lived. We shall illustrate the details of source modulation by describing two other experiments. The first is the classic work of Woods [6] and Dixon and Woods [7] who obtained the first microwave spectrum of a molecular ion, namely CO+. The second is an example of a high-temperature microwave cell for the study of refractory materials. 

We conclude this section on modulation spectrometers by describing a particularly novel and important method, known as velocity modulation, which was originally developed by Gudeman, Begemann, Pfaff and Saykally [10]. It applies specifically to ionic species, and has been used primarily to study the infrared vibration-rotation spectra of molecular ions. If we need an excuse to include it in this book, however, it is provided by Matsushima, Oka and Takagi [11] who used the method to study the far-infrared rotational spectrum of the HeH+ ion. 

Figure 10.14. Velocity modulation spectrometer introduced by Gudeman, Begemann, Pfaff and Saykally [10] to study the infrared vibration-rotation spectra of molecular ions.

Another significant source of variations in the local site energies of molecular ions and excitons in condensed media is the modulation of these energies by the thermal vibrations either of the medium (e.g., acoustical phonons and librons) or of the molecular ion (exciton) itself (intramolecular vibrations). A model Hamiltonian which incorporates electronic interactions with... 

The background problem can be solved by a careful application of the modulated molecular beam technique. Since electron bombardment detection is required for argon and neon, the relative inefficiency must be compensated for by sharp tuning of the ion current and by extended signal integration. 

Gudeman C S and Saykally R J 1984 Velocity modulation infrared laser spectroscopy of molecular ions Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and H L Strauss (Palo Alto, CA Annual Reviews)... 

In crystals with molecular ion groups, like N3, the crystal potential function can be modulated via some of the anharmonic terms in the potential. The effect of this could be a decrease in the barrier height for rotation of the molecular ion with increasing temperature, resulting in the flipping of the molecular ion to another equivalent orientation. The entropy change for such a system is given by the relation... 

Doppler laser-cooling is an essential ingredient in the SCSI-MS technique. First, it provides the necessary damping force to cool directly and sympathetically the atomic and molecular ions, respectively, such that a cold and strongly-coupled two-ion system is formed. Second, it gives rise to the fluorescence photons used in the detection process. Third, the radiation pressure force can be modulated to excite the common motion of the ions. 

Sav Savage, C., Ziurys, L.M. A millimeter/submillimeter velocity modulation spectrometer for studies of molecular ions, Rev. Sci. Instrum. 76 (2005) 043106, p. 1-6. 

Hal Halfen, D.T., Ziurys, L.M. Velocity modulation spectroscopy of molecular ions I The pure rotational spectrum ofTiCt (X r), J. Mol. Spectrosc. 234 (2005) 34-40. 

C.S. Gudeman, R.J. SaykaUy, Velocity modulation infrared laser spectroscopy of molecular ions. Annu. Rev. Phys. Chem. 35, 387 (1984)... 

G. Lan, H.D. ThoU, J.W. Farley, Double-modulation spectroscopy of molecular ions eliminating the background in velocity-modulation spectroscopy. Rev. Sci. Instmm. 62, 944 (1991)... 

M.B. Radunsky, R.J. SaykaUy, Electronic absorption spectroscopy of molecular ions in plasmas by dye laser velocity modulation spectroscopy. J. Chem. Phys. 87, 898 (1987)... 

Fig. 5.11. Left panel Schematic of the pulsed valve discharge modulation source. A 100 kHz square wave discharge 1 kV, lA) is strongly confined in the 1 mmxSOO /am X 4 cm region behind the slit expansion jaws. This yields slabs of spatially modulated jet-cooled radicals and molecular ions for detection via direct IR laser absorption and lock-in detection methods. Right panel Upper trace direct absorption spectra with conventional discharge modulation in a cw discharge source but without lock-in detection Lower trace spectra with frequency modulated discharge and lock-in detection, revealing substantial elimination of radical precursor and improves the absorption sensitivity to the near shot-noise level.

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