Reports of the Academy of Sciences of the USSR
1970. Volume 192, No. 3

UDC 535.370

PHYSICS

S. D. VELIKANOV, S. B. KORMER, V. D. NIKOLAEV, M. V. SINITSYN,
Yu. A. SOLOV’EV, V. D. URLIN

METHOD FOR DETERMINING THE LOWER LIMIT OF THE SPECTRAL WIDTH OF THE LUMINESCENCE LINE OF THE IODINE ATOM TRANSITION \(5^2P_{1/2} - 5^2P_{3/2}\) IN A PHOTODISSOCIATION OKG

(Presented by Academician I. V. Obreimov, 13 XI 1969)

In studying the operation of an OKG, a very important characteristic is the spectral width \(\Delta \lambda\) of the luminescence line of the working transition of the active substance, which is directly related to the gain cross section \(\sigma\) by the known relation (see, for example, \((^1)\))

\[ \sigma = \lambda^4 / 4\pi^2 c \tau \Delta\lambda, \tag{1} \]

where \(c\) is the speed of light, \(\lambda\) is the wavelength of the center of the luminescence line, and \(\tau\) is the radiative lifetime of the excited particles.

The unknown quantity in equation (1) is the spectral width of the luminescence line \(\Delta \lambda\). Below we describe a method for measuring the spectral width of the line of the generated infrared radiation of a photodissociation OKG \((\lambda = 1.315\ \mu)\) with the aid of a Fabry—Perot etalon, whose interference pattern is swept in time by a rotating mirror and recorded by a photodiode.

Fig. 1. 1 — tube with the working substance (internal diameter 12 mm, \(l_t = 320\) mm); 2 — plane resonator mirrors (reflection coefficients \(r_1 = 0.99\), \(r_2 = 0.08\)), length of which is 1 m; 3 — deflecting plate; 4 — diffuser (polyethylene film); 5 — Fabry—Perot etalon (distance between mirrors 20 mm); 6 — objective \((f = 2500\) mm); 7 — rotating mirror (SFR camera); 8 — slit \((0.05\ \text{mm} \times 2\ \text{mm})\); 9 — photodiode (germanium—gold), recording the intensity of the interference maxima; 10 — photodiode (germanium—gold), recording the intensity of the generated radiation

The optical scheme of the setup is shown in Fig. 1. The interference pattern of the Fabry—Perot etalon 5 is localized in the focal plane of objective 6, which in our case coincided with the plane of slit 8. As mirror 7 rotates, the interference maxima pass the slit in succession, and their intensity is recorded by photodiode 9. The sweep speed of the interference pattern was 0.75 mm/μsec. The signals from the photodiodes were fed to a DEO-1 oscillograph, on which, at the same scale,

Fig. 2. In the upper trace, the two right-hand peaks correspond to the interference maxima of the central ring. The lower trace is the generation pulse. The sweep was started at the moment the IFP-5000 flash lamp began to glow. Sweep duration: 100 μsec.

Fig. 3. The signal from photodiode 9 was simultaneously fed to an S1-29 oscilloscope. Figure 3b shows the oscillogram of the first maximum of the central interference ring. Sweep duration: 8 μsec.

the generation pulse and the sequence of interference maxima were recorded in time.

The measurements were carried out in an optical quantum generator with the working substance C$_3$F$_7$I at pressures of 38 and 50 torr. A typical oscillogram at a C$_3$F$_7$I pressure of 38 torr is shown in Fig. 2. In Fig. 3a the central interference ring is superposed on the oscilloscope screen with the recording of the generation process at a C$_3$F$_7$I pressure of 50 torr.

Estimation of the spectral width of the generated-radiation line from the interference pattern ($^2$, $^3$) gives the value $\Delta \lambda_{\Gamma} \simeq 0.04$ Å for both values of the pressure of the working substance.

The interval between resonator modes $\Delta \lambda_p$ for length $L = 1$ m is

\[ \Delta \lambda_p = \lambda^2 / 2L = 0.0086\ \text{Å}. \]

The resonator-mode width $\Delta \lambda_m$, determined in our case by the transmission of the mirrors, is

\[ \Delta \lambda_m = \frac{\lambda^2}{4\pi L}\ln \frac{1}{r_1 r_2} = 0.0035\ \text{Å}. \]

Thus, the spectrum of the generated radiation consists of 4–5 modes. It is not possible to resolve these modes on the oscillogram because of the insufficient resolving power of the photodiode ($^4$) and the Fabry—Perot etalon.

The measured spectral width of the generated radiation $\Delta \lambda_{\Gamma} \simeq 0.04$ Å makes it possible, using equation (1), to obtain an upper limit for the amplification cross section. For the transition of the iodine atom $^2P_{1/2} \to {}^2P_{3/2}$, for which $\tau = 0.13$ sec. ($^5$), we obtain $\sigma \leq 5 \cdot 10^{-18}\ \text{cm}^2$.

REFERENCES

$^1$ A. L. Mikaelyan, M. L. Ter-Mikaelyan, Yu. G. Turkov, Solid-State Optical Generators, Moscow, 1967.
$^2$ A. M. Borbat, I. S. Gorban et al., Optical Measurements, Kiev, 1967.
$^3$ I. M. Nagibina, V. K. Prokof’ev, Spectral Instruments and Spectroscopy Technique, Leningrad, 1967.
$^4$ O. D. Dmitrievskii, B. S. Neporent, V. A. Nikitin, UFN, 64, 3, 447 (1958).
$^5$ I. I. Sobel’man, Introduction to the Theory of Atomic Spectra, Moscow, 1963.