Emission from Excited States

Furthermore, the ultrasonic irradiation of alkanes in the presence of N2 (or NH or amines) gives emission from CN excited states, but not from N2 excited states. Emission from N2 excited states would have been expected if the MBSL originated from microdischarge, whereas CN emission is typically observed from thermal sources. When oxygen is present, emission from excited states of CO2, CH-, and OH- is observed, again similar to flame emission. 

The basic research in our fields is now done largely in universities. It can have incredibly important practical results, but those results cannot normally be predicted in advance. Who would have thought that the basic study of induced energy emission from excited states of atoms and molecules that led to the laser would wind up giving us a better way to record music, or read supermarket prices Would a music company have funded that research Who would have thought that our increased understanding of the chemistry of life would have led to the creation of biotechnology as an entirely new industry The industry that benefited from the basic research could not have funded it, since it did not yet exist. 

The extent of the polarized emission from excited state flu-orophores in a solution can be described in terms of their anisotropy (r), and measuring this can provide insight into the angular displacement of the biomolecule(s) to which the fluorophores are attached (1). Within a homogeneous solution, the ground-state fluorophores are aU oriented randomly. However, when exposed to a polarized excitation source, the molecules with their absorption transition moments oriented... 

Light emission in collisions of He with H2 was studied37 for a He-beam energy of llOOeV. The apparatus has been described in the foregoing section. Emission from excited states of the hydrogen atom was found to be much stronger than emission from excited He-states which could not be detected at all. No emission from excited states of the hydrogen molecule could be found.86,37... 

In its classic form, ECL is regarded as a solution-phase process, on the basis of both direct evidence (Problem 18.4) and the expectation that metal electrodes quench excited states (18, 19). The band structure of semiconductor electrodes sometimes removes the latter difficulty (see Section 18.2), and emission from excited states produced directly in heterogeneous charge transfer at semiconductors can occur (20-22). More recently, even surface films, such as monolayer assemblies and polymer-modified electrodes (Chapter... 

The line shape function is given for transition in emission from excited state s) to a ground state 0) in the form [92]... 

Figure Al.6.21. Bra and ket wavepacket dynamics which detennine the coherence overlap, (( ) ( ) ). Vertical arrows mark the transitions between electronic states and horizontal arrows indicate free propagation on the potential surface. Full curves are used for the ket wavepacket, while dashed curves indicate the bra wavepacket. (a) Stimulated emission, (b) Excited state (transient) absorption (from [41]).

Multiple-Bubble Sonoluminescence. The sonoluminescence of aqueous solutions has been often examined over the past thirty years. The spectmm of MBSL in water consists of a peak at 310 nm and a broad continuum throughout the visible region. An intensive study of aqueous MBSL was conducted by VerraH and Sehgal (35). The emission at 310 nm is from excited-state OH, but the continuum is difficult to interpret. MBSL from aqueous and alcohol solutions of many metal salts have been reported and are characterized by emission from metal atom excited states (36). 

Sonoluminescence from nonaqueous Hquids has only recentiy been examined. Flint and SusHck reported the first MBSL spectra of organic Hquids (37). With various hydrocarbons, the observed emission is from excited states of (d Ilg — 11, the Swan lines), the same emission seen in flames. 

Spectroscopic Probes of Cavitation Conditions. Determination of the temperatures reached ia a cavitating bubble has remained a difficult experimental problem. As a spectroscopic probe of the cavitation event, MBSL provides a solution. High resolution MBSL spectra from sUicone oU under Ar have been reported and analy2ed (7). The observed emission comes from excited state has been modeled with synthetic spectra as a... 

White Phosphorus Oxidation. Emission of green light from the oxidation of elemental white phosphoms in moist air is one of the oldest recorded examples of chemiluminescence. Although the chemiluminescence is normally observed from sotid phosphoms, the reaction actually occurs primarily just above the surface with gas-phase phosphoms vapor. The reaction mechanism is not known, but careful spectral analyses of the reaction with water and deuterium oxide vapors indicate that the primary emitting species in the visible spectmm are excited states of (PO)2 and HPO or DPO. Ultraviolet emission from excited PO is also detected (196). 

Although protons and neutrons are not emitted from the ground states of these isotopes, there are many cases where particles are emitted from excited states. For example, Cs decays by electron capture and -emission to excited levels ia and ia 7% of these cases protons are emitted from... 

Since an atom of a given element gives rise to a definite, characteristic line spectrum, it follows that there are different excitation states associated with different elements. The consequent emission spectra involve not only transitions from excited states to the ground state, e.g. E3 to E0, E2 to E0 (indicated by the full lines in Fig. 21.2), but also transisions such as E3 to E2, E3 to 1( etc. (indicated by the broken lines). Thus it follows that the emission spectrum of a given element may be quite complex. In theory it is also possible for absorption of radiation by already excited states to occur, e.g. E, to 2, E2 to E3, etc., but in practice the ratio of excited to ground state atoms is extremely small,... 

Eor example, street lamps use the emissions from excited sodium atoms, the dazzling colors of a fireworks display come from photons emitted by metal ions in excited states, and the red light in highway flares often comes from excited Sr ions. 

One of the first explanations for the afterglow of organic compounds in rigid solutions after exposure to UV light was offered by Perrin.<12) Perrin postulated that the excited molecules could undergo a transition to a metastable state of lower energy. Emission from this state was thought to be... 

The ratio of the intensity of anti-Stokes and Stokes lines is primarily determined by the Boltzmann population of the excited vibrational states. For mid-IR frequencies this fractional population is very low (seIO-4 at 2000cm-1). As a result, Raman spectra are usually taken from the Stokes side of the Rayleigh line as these are generally very much more intense and are not broadened by emissions from hot states. 

The sample is continuously irradiated and the fluctuations in the fluorescence intensity arise due to any event which makes the fluorophore unavailable to be excited to the emissive singlet excited state, such as diffusion of the fluorophore out of the detection volume, formation of a dark state, such as a triplet excited state, or photoreaction. The concentration of fluorophore in the detection volume has to be low (10 13—10 8M) so that the fluctuation in the intensity for one molecule is observable over any background emission. The high concentration limit is a consequence of the fact that the correlated photons from single molecules scale with the number of molecules in the detection volume, while the contribution from uncorrelated photons, arising from the emission from different molecules, scales with the square of the number of molecules. The lowest concentration is determined by the probability of finding a molecule in the detection volume.58... 

If an atomic transition is optically pumped by a beam of laser radiation having the appropriate frequency, the population in the upper state can be considerably enhanced along the path of the beam. This causes an intensification of the spontaneous emission from this state, which contains information about the conditions within the pumped region, since the exponential decay time for the intensified emission depends upon both the electron number density and the electron temperature. The latter can be obtained from the intensity ratio of the fluorescence excited from two different lower levels, if local thermal equilibrium is assumed. This method has been dis-... 

The emission from molecular halogens in the 3n0u+ state resulting from shock heating could, in principle, arise from both inverse predissociation and direct recombination on account of the large thermal populations of the excited atoms at these high temperatures. Emission from this state has been observed hitherto at high temperatures from iodine, bromine, and chlorine... 

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