Optical excitations

Optical excitation differs from collisional excitation in a fundamental way the exciting photon is absorbed by the target atom. As a result, specifying the energy 

In most optical excitations the resolution is determined by the Doppler effect or the finite linewidth of the light source. The Doppler effect gives a typical frequency width of 1 GHz, and the width of the light source can be anywhere from 1 kHz to 30 GHz. We assume that these widths are larger than the radiative width. The photoionization cross sections from the ground states of H, alkali, and the alkaline earth atoms are given in Table 3.3. 20 

A typical example of the pulsed dye laser excitation is the beam experiment shown in Fig. 3.4 in which Na atoms in a thermal beam are excited in two steps from the ground state 3s to the 3p state with a yellow dye laser photon and from the 3p state to a high lying ns or nd state with a second, blue photon.21 

Using continuous wave (cw) laser excitation it is possible to excite atoms with substantially higher efficiency than using pulsed lasers. For example a single mode laser of 1 MHz linewidth has a resolution 3 x 104 better than the pulsed laser 

Purely optical excitation is possible for alkali and alkaline earth atoms. For most other atoms the transition from the ground state to any other level is at too short a wavelength to be useful. To produce Rydberg states of such atoms a combination of collisional and optical excitation is quite effective. A good example is the study of the Rydberg states of Xe by Stebbings et al.24 As shown in Fig. 3.5, a thermal beam of Xe atoms is excited by electron impact, and a reasonable fraction of the excited atoms is left in the metastable state. Downstream from the electron excitation the atoms in the metastable state are excited to a Rydberg state by pulsed dye laser excitation. 

---

Electronic and optical excitations usually occur between the upper valence bands and lowest conduction band. In optical excitations, electrons are transferred from the valence band to the conduction band. This process leaves an empty state in the valence band. These empty states are called holes. Conservation of wavevectors must be obeyed in these transitions + k = k where is the wavevector of the photon, k is the... 

Figure Al.6.26. Stereoscopic view of ground- and excited-state potential energy surfaces for a model collinear ABC system with the masses of HHD. The ground-state surface has a minimum, corresponding to the stable ABC molecule. This minimum is separated by saddle points from two distmct exit chaimels, one leading to AB + C the other to A + BC. The object is to use optical excitation and stimulated emission between the two surfaces to steer the wavepacket selectively out of one of the exit chaimels (reprinted from [54]).

Larger molecules generally caimot be studied in quite the same way, as an electric discharge merely breaks them up into smaller molecules or atoms. In such a case excited states are usually produced by optical excitation using light of the same or higher energy. Many modem fluorimeters are made with two... 

Shank C V, Yen R and Hirlimann C 1983 Femtosecond-time-resolved surface structural dynamics of optically excited silicon Phys. Rev. Lett. 51 900-2... 

Figure B2.3.13. Model 2-level system describing molecular optical excitation, with first-order excitation rate constant W 2 proportional to the laser power, and spontaneous (first-order rate constant 21) stimulated (first-order rate constant 1 2 proportional to the laser power) emission pathways.

The velocity distribution/(v) depends on the conditions of the experiment. In cell and trap experiments it is usually a Maxwell-Boltzmann distribution at some well defined temperature, but /(v) in atomic beam experiments, arising from optical excitation velocity selection, deviates radically from the nonnal thennal distribution [471. The actual signal count rate, relates to the rate coefficient through... 

CL is a powerful tool for the characterization of optical properties of wide band-gap materials, such as diamond, for which optical excitation sources are not readily available. 

As mentioned earlier, CL is a powerful tool for the characterization of optical properties of wide band-gap materials, such as diamond, for which optical excitation sources are not readily available. In addition, electron-beam excitation of solids may produce much greater carrier generation rates than typical optical excitation. In such cases, CL microscopy and spectroscopy are valuable methods in identifying various impurities, defects, and their complexes, and in providing a powerful means for the analysis of their distribution, with spatial resolution on the order of 1 pm and less. ... 

Fig. 7. Calculated optical excitation spectra (left) and exciton absorption spectra (right) of a semiconducting CNT for a parallel polarisation.

Figure 10-8. Emission spectra of a free standing film of a blend system consisting of 0.9% MEH-PPV in polystyrene with ca. I011 cm 3 TiOj-particlcs. The nanoparlicles act as optical scattering centers. The emission spectrum is depicted for two different excitation pulse energies. Optical excitation was accomplished with laser pulses of duration I Ons and wavelength 532 nm (according to Ref. 171).

Figure 10-12. Lcfi hand side Slruclure of a PPV microcavily. A thin film of ihe conjugated polymer is deposited on top of a highly reflective distributed Bragg refieclor (DBR). The second mirror is then fabricated by evaporation of a silver layer. Right hand side Emission spectra of the microcavily at excitation cnetgics or 0.0S pJ (dashed line) and l. l pJ (solid line), respectively. Laser pulses ol duration 200-300 ps and a wavelength of 355 nm were used for optical excitation (according to Ref. [39]).

Techniques other than UV-visible spectroscopy have been used in matrix-isolation studies of Ag see, for example, some early ESR studies by Kasai and McLeod 56). The fluorescence spectra of Ag atoms isolated in noble-gas matrices have been recorded (76,147), and found to show large Stokes shifts when optically excited via a Si j — atomic transition which is threefold split in the matrix by spin-orbit and vibronic interactions. The large Stokes shifts may be explained in terms of an excited state silver atom-matrix cage complex in this... 

Homola, J., "Thin Films Study by Means of Optically Excited Surface Plasmons, Nano 94—International Coirference on Nanometrology Scanning Probe Microscopy and Related Techniques, Proceedings, Nano 94, Bmo, Czech Rep., 1994, pp. 84-87. 

It has been reported that while in aqueous solution the lifetime of optically excited nile red is 0,65 ns inside AOT-reversed micelles it is 3,73 ns, becoming 2,06 ns at/ = 30 [150],... 

How must this theory be modified to describe the effect of the optical excitation The incident electric and magnetic X-ray fields are now pulses Ex(r, t) = Exo(t) exp[j(q r - Oxt)] and Hx(r, t) = Hxo(t) exp[/(q/r - Oxt)]. They still are plane waves with a carrier frequency Ctx, but their amphtudes Exo(t) and Hxo(t) vary with time. The same statement applies to the electron density n r, t), which also is time dependent. However, these variations are all slow with time scales on the order of 1/Ox, and one can neglect 5Exo(0/ 8Hxo(t)/8t as compared to iOxExo(t) and iTlxHxo(0- Detailed calculations then show that [17]... 

The purpose of this section is to describe recent achievements in time-resolved X-ray diffraction from liquids. Keeping the scope of the present chapter in mind, neither X-ray diffraction from solids nor X-ray absorption will be discussed. The majority of experiments realized up to now were performed using optical excitation, although some recent attempts using infrared excitation were also reported. The main topics that have been studied are (1) visualization of atomic motions during a chemical reaction, (2) structure of reaction intermediates in a complex reaction sequence, (3) heat propagation in impulsively heated liquids, and (4) chemical hydrodynamics of nanoparticle suspensions. We hope that the actual state-of-the-art will be illustrated in this way. 

Relaxation of Thermally Quenched or Optically Excited High-Spin States Resulting... 

---