Excitations, detection processes

One of the main advantages of FT spectrometers is that, since the FID is in digital form, we can repeat the excitation/detection process a number of times and all the resulting scans can be added and the FT performed on the resultant FID. In this way, we can improve the signal-to-noise ratio and can detect nuclei which are not very abundant (e.g. C) or have low sensitivity to NMR (see Section 4.2). These nuclei could not have been detected on the older continuous wave instruments, as the spectrum was the result of a single scan, obtained as one of the frequency or magnetic held were varied while keeping the other constant. 

Domcke W, Stock G (1997) Theory of ultrafast nonadiabatic excited-state processes and their spectroscopic detection in real time. Adv Chem Phys 100 1—170... 

D. Lynch, J. F. Endicott. A Pulsed Photoacoustic Microcalorimeter for the Detection of Upper Excited-State Processes and Intersystem Crossing Yields. Appl. Spectroscopy 1989, 43, 826-833. 

The photolytic and probe pulses are colinear when they reach the sample. The photolytic pulse produces excited states and photofragments, and the probe pulse which follows closely behind must be used to analyse the concentration and/or the chemical nature of the transients. The major detection processes are known as laser-induced fluorescence (LIF) and multiphoton ionization (MPI). Transient absorptions can also be used in some cases, and this is similar to ps spectroscopy. 

W. Domcke and G. Stock, in Adv. Chem. Phys., I. Prigogine and S. A. Rice, Eds., Wiley, New York, 1997, Vol. 100, pp. 1-169. Theory of Ultrafast Nonadiabatic Excited-State Processes and Their Spectroscopic Detection in Real Time. 

In this chapter, we review important concepts regarding vibrational spectroscopy with the STM. First, the basis of the technique will be introduced, together with some of the most relevant results produced up to date. It will be followed by a short description of experimental issues. The third section introduces theoretical approaches employed to simulate the vibrational excitation and detection processes. The theory provides a molecular-scale view of excitation processes, and can foresee the role of various parameters such as molecular symmetry, adsorption properties, or electronic structure of the adsorbate. Finally, we will describe current approaches to understand quenching dynamics via internal molecular pathways, leading to several kinds of molecular evolution. This has been named single-molecule chemistry. 

Figure 17 Schematic representation of the Fourier transform ion cyclotron resonance (FT-ICR) excitation and detection process.

Early semiempirical calculations laid the foundations for subsequent ab initio methods which can now not only describe the electronic structure of optically accessible excited states, but also model the wavepacket propagation on the resulting potential energy surfaces. These models are supported by ultrafast studies using femptosecond (fs) pulsed lasers with a variety of detection systems. Many systems use indirect detection of excited-state processes because many excited states are unbound and not amenable to spectroscopic techniques. 

The most notable feature of the process described above is that ions are not destroyed by the detection process. Therefore ions can be further manipulated after detection. The simplest example is the remeasurement of ions (i.e., repeating the excite and/or detect sequence to obtain another mass spectrum of the same group of ions). More complex manipulations include multiple stages of MS/MS. The facts that ions are detected nondestructively and that they are trapped in a region of space mean that very complex sequences of ion manipulations are possible, making FT-ICR instruments the most versatile of all mass spectrometers. 

We checked that the principal detectable process occurring in the sample was excitation of bromide rather than other chromophores. 

With FTICR, all ions produced (or isolated) in the ICR cell are detected simultaneously, as has been explained in Section 1.3.2.2. The time domain signal, i.e., the image current produced at the receiver plates by the orbiting ions (cf. Figures 1.24 and 1.25), can readily be amplified to yield a measurable signal moreover, the ions are not destroyed during the detection process and, thus, can be re-excited and remeasured if an improvement of the signal... 

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