Excitation of levels

Fig. 11.8). The resonances associated with fission appear to cluster in bunches. Not all resonances in the compound nucleus lead to fission. We can understand this situation with the help of Figure 11.9. The normal resonances correspond to excitation of levels in the compound nucleus, which are levels in the first minimum in Figure 11.9. When one of these metastable levels exactly corresponds to a level in the second minimum, then there will be an enhanced tunneling through the fission barrier and an enhanced fission cross section. 

Fig. 21. Potential function of hydrogen bonds in DNA and possible method for excitation of levels by IR laser radiation to stimulate proton tunneling79 ...

The reaction Bi ( He ) Be has been observed with protons of 3.4 MeV (Craig et al ). Both this reaction and the B ( a) reaction have been studied by Reynolds using 18 MeV protons and a differential proportional counter for discriminating against long range scattered protons. The excitation of levels in Be at 0.49, 4.72, 6.27, 7.21 and 14.6 MeV was indicated. 

Fig. 7.9 (a) Level scheme illustrating the coherent excitation of levels 11) and 2) by a short pulse, (b) Quantum beats observed in the fluorescence decay of two coherently excited levels. Insert Fourier spectrum l o)) of (b) with o) 2 = Ejh... 

In conclusion, the field of rare earth lasers is mature, but it is not exhausted. Additional laser schemes and materials will undoubtedly be exploited. For example, there are 1639 free-ion energy levels associated with the 4f" configurations of the thirteen trivalent lanthanide ions. Yet, of the 192 177 possible transitions between pairs of levels, only 34 have been used for lasers. It is certain that, given suitable pump sources and materials, stimulated emission involving many more transitions can be obtained. This is particularly true with the increasing availability of lasers at new wavelengths for pump sources and of tunable lasers for selective excitation of levels. 

Fig.11.23a,b. Quantum beat spectroscopy, (a) Level scheme illustrating coherent excitation of levels 1 and 2 with a short broad-band pulse, (b) Fluorescence intensity showing a modulation of the exponential decay... 

The first mfonnation on the HE vibrational distribution was obtained in two landmark studies by Pimentel [39] and Polanyi [24] in 1969 both studies showed extensive vibrational excitation of the HE product. Pimental found that tire F + H2 reaction could pump an infrared chemical laser, i.e. the vibrational distribution was inverted, with the HF(u = 2) population higher than that for the HF(u = 1) level. A more complete picture was obtained by Polanyi by measuring and spectrally analysing tlie spontaneous emission from vibrationally excited HE produced by the reaction. This infrared chemiluminescence experiment yielded relative populations of 0.29, 1 and 0.47 for the HF(u =1,2 and 3)... 

A refinement of the ENDOR experiment is electron-nnclear-nnclear triple resonance, now commonly denoted TRIPLE. In TRIPLE experiments one monitors the effect of a simnltaneons excitation of two nnclear spm transitions on the level of the EPR absorption. Two versions, known as special TRIPLE (ST) and general TRIPLE (GT), are rontinely perfonned on connnercially available spectrometers. 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

B) The multiphoton excitation of electronic levels of atoms and molecules with visible or UV radiation generally leads to ionization. The mechanism is generally a combination of direct, Goeppert-Mayer, and quasi-resonant stepwise processes. Since ionization often requires only two or tln-ee photons, this type of multiphoton excitation is used for spectroscopic purposes in combination with mass-spectrometric detection of ions. 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

NH in its v = 1 vibrational level and in a high rotational level (e.g. J> 30) prepared by laser excitation of vibrationally cold NH in v = 0 having high J (due to nahiral Boltzmann populations), see figure B3.T3 and... 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

All teclmologically important properties of semiconductors are detennined by defect-associated energy levels in the gap. The conductivity of pure semiconductors varies as g expf-A CgT), where is the gap. In most semiconductors with practical applications, the size of the gap, E 1-2 eV, makes the thennal excitation of electrons across the gap a relatively unimportant process. The introduction of shallow states into the gap through doping, with either donors or acceptors, allows for large changes in conductivity (figure C2.16.1). The donor and acceptor levels are typically a few meV below the CB and a few tens of meV above the VB, respectively. The depth of these levels usually scales with the size of the gap (see below). 

Osgood R M Jr, Sackett P B and Javan A 1974 Measurement of vibrational-vibrational exchange rates for excited vibrational levels (2 v 4) in hydrogen fluoride J. Chem. Phys. 60 1464-80... 

When the states P1 and P2 are described as linear combinations of CSFs as introduced earlier ( Fi = Zk CiKK), these matrix elements can be expressed in terms of CSF-based matrix elements < K I eri IOl >. The fact that the electric dipole operator is a one-electron operator, in combination with the SC rules, guarantees that only states for which the dominant determinants differ by at most a single spin-orbital (i.e., those which are "singly excited") can be connected via electric dipole transitions through first order (i.e., in a one-photon transition to which the < Fi Ii eri F2 > matrix elements pertain). It is for this reason that light with energy adequate to ionize or excite deep core electrons in atoms or molecules usually causes such ionization or excitation rather than double ionization or excitation of valence-level electrons the latter are two-electron events. 

Two typical dye molecules. The europium complex (a) transfers absorbed light to excited-state levels of the complexed Eu , from which lasing occurs. The perylene molecule (b) converts incident radiation into a triplet state, which decays slowly and so allows lasing to occur. 

---