Collision-induced emission

The spectra of liquids and solids cannot be studied here in any detail we refer the reader to other works, such as the two volumes by Gray and Gubbins, and the review articles by Guillot and Birnbaum (1989). 

Dipole moments that absorb will also emit radiation. Especially at elevated temperatures collision-induced emission (CIE) spectra may be expected and have in fact been found in the laboratory using shock tubes. Stellar atmospheres are also known to emit radiation by interatomic (e.g., H-He) and intermolecular (H2-H2, etc.) interactions. 

Shock tubes consist of a driver and a test section. The driver is pressurized with gas until the diaphragm, which separates driver and 

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Collision-induced emission. Emission spectra of ordinary atoms and molecules correspond to downward transitions, from an initial energy level higher than the final one, whereas absorption involves the inverse transition. Both exist in supermolecules as well and have recently been seen in shocktube studies. Emission spectra are generally much richer than absorption spectra and may include hot bands, which involve transitions between excited vibrational (or electronic) states [116]. 

Collision-induced emission. Induced spectra are usually studied in absorption. However, emission spectra of supermolecular nature will also interest us here, for example, in connection with shock tube measurements and astrophysical studies (stellar and planetary atmospheres). 

Fig. 3.49. Collision-induced emission in the fundamental band of hydrogen after [116]. 20% hydrogen in argon at the temperature T = 2844 K.

We note that the collision-induced emission profile may be obtained from the spectral density, J(co), according to... 

Experimental data of photon-assisted collisions are much sparser than those of collision-induced absorption. To some extent, this fact may be due to the greater difficulty in working with excited states [176]. There was an early hope that collision-induced emission or LICET processes could result in new laser sources. When these methods did not work well for various reasons, some of the interest in these phenomena waned. 

R. Krech, G. Caledonia, S. Schertzer, K. Ritter, T. W. Wilkerson, L. Cotnoir, R. Taylor, and G. Birnbaum. Laboratory observation of collision induced emission in the fundamental vibration rotation band of H2. Phys. Rev. Lett., 49 1913, 1982. 

The intensity of the collision induced emission follows the order Xe > Kr > Ar > N2 > H2 > He (349, 436). According to Cunningham and Clark (254) this molecular emission is 40 , of the total quenching process in the case of Xe, but it is only V / in the case of Ar. From the Stcrn-Volmcr type plot of the intensity ratio of /y (with added gas) to / (without added gas) against the pressure of an added gas, Black et al. (115) have recently obtained the rate constant for induced emission by He, Ar, N2, H2, Kr, and Xe. 

Unfortunately, two-photon excitation of hydrogen 1S-2S requires ultraviolet radiation near 243 nm, where there are still no good cw sources available. The best spectra so far have been recorded by C. Wieman, (39) who used a cw dye laser near 486 nm followed by a pulsed dye laser amplifier and a crystal frequency doubler. Discharge-produced hydrogen atoms were excited with a standing wave field from this source, and the signal was detected by monitoring the collision-induced emission of vacuum ultraviolet Lyman-a photons. 

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