Absorption quantum theory

The quantum theory of spectral collapse presented in Chapter 4 aims at even lower gas densities where the Stark or Zeeman multiplets of atomic spectra as well as the rotational structure of all the branches of absorption or Raman spectra are well resolved. The evolution of basic ideas of line broadening and interference (spectral exchange) is reviewed. Adiabatic and non-adiabatic spectral broadening are described in the frame of binary non-Markovian theory and compared with the impact approximation. The conditions for spectral collapse and subsequent narrowing of the spectra are analysed for the simplest examples, which model typical situations in atomic and molecular spectroscopy. Special attention is paid to collapse of the isotropic Raman spectrum. Quantum theory, based on first principles, attempts to predict the. /-dependence of the widths of the rotational component as well as the envelope of the unresolved and then collapsed spectrum (Fig. 0.4). 

In quantum theory as well as in classical theory, linear absorption of light at frequency co is described by a spectral function... 

By comparison, the only variable measured in absorption spectroscopy is transmission as a function of incident wavelength. Once quantum theory had revealed the potential... 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

A time-independent quantum mechanical study by Engel and coworkers (1985) gave qualitatively the same results as the time dependent analog and the quassi classical trajectory method. A quantum theory for probing transition-state absorption/emission has also been developed by Lee et al. (1989). 

According to the quantum theory, light is also quantised. The absorption or emission of light occurs by the transfer of energy as photons. These photons have both wavelike and particle-like properties and each photon has a specific energy, E, given by Planck s law ... 

H. B. Levine. Quantum theory of collision-induced absorption in rare-gas mixtures. Phys. Rev., 160 159, 1967. 

There are several reasons for starting this account with a discussion of electromagnetic radiation. Historically, it was in this area that the quantum theory first developed. It is easier here to understand the evidence for the theory, and to appreciate some of its paradoxical consequences, than it is in the quantum theory of matter. The applications of the light-quantum hypothesis, as it was first called, also provide key pieces of evidence for the quantization of energy in atoms and molecules. Studies of the absorption and emission of radiation—the field of spectroscopy—and of the effect of light on chemical reactions—photochemistry—are very important areas of modem chemistry, in which the quantum nature of radiation is crucial. 

A particularly fruitful way into the study of chemical dynamics is through the study of photochemistry, the set of reactions caused by the absorption or emission of light. Chemical bonds can be broken by the absorption of energy-in the form of light. Thanks to quantum theory, we now have a far deeper understanding of photochemical reactions than was previously possible. And, thanks to some extraordinary developments in instrumentation and laboratory technique in recent decades, we now have a good deal of empirical knowledge of chemical events that were previously completely beyond the reach of experiment. 

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