Raman shifts of Si nanocrystals versus size were studied theoretically by a bond polarizability model. Zero-dimensional spheres and one-dimensional columns were considered. The relation between the Raman shift and the size for Si spheres and columns was established, from which the size of Si nanocrystals can be obtained for a given Raman shift or vice versa.

The ionization trace rises sharply in less than 20 nsec, then decays in several hundred nanoseconds.The minima in the breakdown curves are predicted by the familiar theory of electron impact ionization. The change in energy of the electrons is given by de/dt =e 2 EQ 2 v m /2m x (v m 2 + u) 2 ), where E 0 and oo are the amplitude and frequency of the light wave, v m is the electron momentum-transfer collision frequency with neutrals, and e and m are the charge and mass of the electron. This energy change has a maximum when v m =a>. The collision frequency v m is related to pressure p (mm Hg) by v =p p v = ($Axl0 7 )P u 1/2 p,(1) m 0 c c where p 0 = (213/T)p is the reduced pressure, P c is the collision probability, v is the velocity, and U is the mean energy in electron volts. The approximate range of energy U is from the thermal energy, 0.04 eV, to the ionization potential, 24.5 eV for He, 15.7 eV for Ar, and 15.5 eV for N 2 . Data are readily available from the literature giving P c in terms of U 1/2 for most gases. For He the product U in P c changes very little for U between 4 and 25 eV. Using a value in this range and setting v m = a> = 2.72 x 10 15 sec" 1 in Eq. (1) gives p = 21 400 psi for the pressure at which the minimum in the breakdown curve should occur. The same procedure for Ar predicts the minimum to be at p = 3300 psi. These results are in agreement with the experimental data presented, being quite close for Ar and within a factor of 2 for He. In N 2The frequency dependence of the threshold intensities for the breakdown of gases by optical maser radiation has been of interest in attempting to determine the fundamental energy coupling mechanisms responsible for the breakdown phenomenon. Investigations by the authors 1 using 1.06/i radiation from a Nd-inglass and 0.69/i radiation from a ruby optical maser led to the conclusion that the threshold intensity for breakdown increases with decreas-interpretation of the results must take into account the low-level inelastic collisional processes prevailing as well as the elastic.It should be noted that the curve of P c vs U in for He has a very broad maximum. The minimum in the curve of threshold E versus pressure is correspondingly broad. For Ar and N 2 the P c maxima are much sharper, and correspondingly so are the threshold minima.In conclusion, minimum breakdown fields have been observed for laser-induced discharges. These minima are characteristic of electronimpact ionization where electron heating occurs through energy transfer from the light wave to the electrons undergoing collisions with neutrals. The presence, pressure, and sharpness of these minima are predicted by a simple electron-impact ionization theory, and these predictions agree with the experimental data presented here. ].ing wavelength. A similar observation was made by Haught, Meyerand, and Smith in He, Ar, and air 2 at the same maser frequencies, and by Akhmanov et al. 3 using the Nd radiation and its second harmonic in air. We have made further studies of breakdown thresholds in resea...

Calculations of the transition frequencies and absorption coefficients of microwave rotational transitions are given for a number of atmospheric pollutants and constituents. New measurements of the absorption coefficients are made in the vicinity of 70 GHz. The apparatus •used in these measurements is briefly described. The calculated absorption coefficients are compared with these measurements and with existing measurements at other frequencies where available. Transitions with frequencies up to about 200 GHz are considered for the molecules and radicals so 2 , o 3 , H 2 o, N0 2 , H 2 S, H 2 co, NH 3 , CO, OCS, N 2 0, NO, OH, o 2 , SO. Also discussed are criteria for the selection of appropriate transitions for the development of high sensitivity monitors to be used in air pollution and combustion research.

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