Broadening thermal

A wide variety of different mechanisms may participate in the PT process and influence the interpretadon of a spectrum. At room temperature, PL emission is thermally broadened. As the temperature is lowered, features tend to become sharper, and PL is often stronger due to fewer nonradiadve channels. Low temperatures are typically used to study phosphorescence in organic materials or to identify particular impurides in semiconductors. 

K, to optimize the energy resolution by reducing the contribution of thermal broadening to the line-width, and lead is, of course, superconducting at that temperature. 

Recoil Energy Loss in Free Atoms and Thermal Broadening of Transition Lines... 

We have already pointed out that the breadth of rib in HD0/D20(as) is greater (—115 cm-1) than that of rib in H20(as). In addition to the proposed overlapping of the bands nb (HOD) and rbb (HOD), we must allow that any difference in hydrogen bonding character between OH... 0 and OD... 0 will also contribute to the breadth of the distribution of local environments, hence also to the breadth of the transition. Presumably any such contribution to variation in the local environments is in addition to the effects already present in H20(as). Of course, at the temperatures used by VRB, thermal broadening is negligibly small relative to the broadening from the other sources mentioned. 

Thermal broadening effects of the density of states should also be taken into account yielding... 

Figure 5. (a) Thermal broadening function F for both electrodes normal, and for... 

The observed second harmonic signal is the double convolution of the oscillator density of states function Ds(e) with a thermal broadening function F(eVQ+E-h 0 ( Fig.5a ) and a modulation broadening function G(E) ( Fig.Sb ). 

Equation (7.22) is at the heart of spectroscopy. The positions of the absorption lines reflect the energy levels of the excited complex and the widths provide information about the lifetime and therefore about the coupling to the continuum states. The latter requires, however, that the measured widths are the true homogeneous line widths, i.e., unadulterated by poor resolution and/or thermal broadening, for example. Each resonance has a characteristic width. In Chapters 9 and 10 we will discuss how the final fragment distributions reflect the initial state in the complex and details of the fragmentation mechanism. 

Fig. 7.12. Comparison of the measured and the calculated absorption spectra for the So — Si transition in CH3ONO. The quantum number n denotes vibrational excitation of the NO moiety in the complex. The theoretical spectrum is obtained in a three-dimensional wavepacket calculation including the ONO bending angle in addition to the two N-0 stretching coordinates. The spectrum is convoluted with a Gaussian function with width AEres = 0.02 eV in order roughly to mimic thermal broadening and is artificially shifted along the energy axis. Reproduced from Untch, Weide, and Schinke (1991a).

As in Chapter 9 we discuss first the elastic limit (no exit channel excitation) in Section 10.1 and subsequently the more interesting inelastic case in Section 10.2. In Section 10.3 we consider the decay of long-lived resonance states and the impact of exit channel dynamics on the product distributions. A simple approximation, the so-called impulsive model, which is frequently employed to analyze experimental distributions in the absence of a PES, is discussed critically in Section 10.4. The chapter ends with a more qualitative assessment of thermal broadening of rotational state distributions in Section 10.5... 

A rigorous modelling of thermal broadening is — in practice — quite cumbersome and tedious. Let us consider a general asymmetric top molecule such as H2O, for example. Each total angular momentum state, specified by the quantum number J, splits into (2 J + 1) nondegenerate substates with energies E 0f (K = 1,..., 2J + 1). Every one of these (2J + 1) rotational states corresponds to a different type of rotational motion and is described by a distinct rotational wavefunction (see Section 11.3). 

Thermal broadening of an electronic transition results from the population of additional vibrational levels of the electronic ground state illustrated by the potential energy diagram in fig. 3.14. Conversely, spectral features may be narrowed, and better resolution of absorption bands achieved, by performing crystal field spectral measurements at low temperatures. Under these conditions, vibrational peaks may contribute to fine structure observed on electronic absorption bands of transition metal-bearing phases, particularly in low temperature spectra. 

Acoustical phonons are important in the thermal broadening of the h-polarized exciton of anthracene. 

Observation of the spectra at low temperatures, with an upper limit of 80 K. As discussed in Section II.C, the surface structures follow the same thermal broadening as the bulk 0-0 structures. This provides three critical temperatures—T, < 77 K, Tn < 30 K, and T, < 5 K—as the energy gap between the surface and the bulk structures decays from 200 to a few reciprocal centimeters see Fig. 3.1. 

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