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Physical Review

Transition Probabilities and Quenching in the 3P State of Sodium

N. E. Berry, G. K. Rollefson

Published Nov 1, 1931• 5 citations
Publisher

Abstract

The effect of foreign gases upon the fluorescence of sodium vapor excited by the second line of the principal series has been investigated. The gases used were helium, nitrogen, and hydrogen and the intensities of both the 3303A line and the $D$ light were observed. From the variation in intensity of the 3303A line as gas is added it is concluded that the presence of an inert gas has no appreciable effect upon the relative transition probabilities in the ${3}^{2}P$ state of sodium. With nitrogen or hydrogen as the added gas the $D$ light intensity variation followed that of the 3303A line very closely but with helium a marked increase in intensity was produced by the first gas added with a decrease at higher pressures. This increase was explained as due to a transfer of sodium atoms from the ${3}^{2}P$ to the ${3}^{2}D$ state by collisions, thus increasing the number of atoms which radiate the $D$ light. In addition to this increase in number the collisional transfers produced an increased "Doppler width" in the $D$ light which persisted up to pressures well over 100 mm of helium and caused a decreased absorption of the fluorescence light. The bearing of this work on the difference in the intensity ratio of the ${2}^{2}S\ensuremath{-}{3}^{2}P$ and ${1}^{2}S\ensuremath{-}{3}^{2}P$ transitions as reported by Weiss for an arc and by Christensen and Rollefson in fluorescence is discussed. It is concluded on the basis of experimental evidence that any explanation of the difference which depends upon multiplicity of the ${3}^{2}P$ state is incorrect. An explanation of this difference based upon the assumption that the life period of an excited atom varies inversely as the transition probability of the radiation emitted is presented. It is shown that recent work on the life periods of atoms in the ${2}^{3}{S}_{1}$ state of mercury at least partially supports this assumption. A new type of sodium arc with increased intensity and stability of operation is described.

DOI
10.1103/physrev.38.1599

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•Jan 1, 1967•0 citations
The effect of foreign gases upon the fluorescence of sodium vapor excited by the second line of the principal series has been investigated. The gases used were helium, nitrogen, and hydrogen and the intensities of both the 3303A line and the DDD light were observed. From the variation in intensity of the 3303A line as gas is added it is concluded that the presence of an inert gas has no appreciable effect upon the relative transition probabilities in the 32P{3}^{2}P32P state of sodium. With nitrogen or hydrogen as the added gas the DDD light intensity variation followed that of the 3303A line very closely but with helium a marked increase in intensity was produced by the first gas added with a decrease at higher pressures. This increase was explained as due to a transfer of sodium atoms from the 32P{3}^{2}P32P to the 32D{3}^{2}D32D state by collisions, thus increasing the number of atoms which radiate the DDD light. In addition to this increase in number the collisional transfers produced an increased "Doppler width" in the DDD light which persisted up to pressures well over 100 mm of helium and caused a decreased absorption of the fluorescence light. The bearing of this work on the difference in the intensity ratio of the 22S\ensuremath−32P{2}^{2}S\ensuremath{-}{3}^{2}P22S\ensuremath−32P and 12S\ensuremath−32P{1}^{2}S\ensuremath{-}{3}^{2}P12S\ensuremath−32P transitions as reported by Weiss for an arc and by Christensen and Rollefson in fluorescence is discussed. It is concluded on the basis of experimental evidence that any explanation of the difference which depends upon multiplicity of the 32P{3}^{2}P32P state is incorrect. An explanation of this difference based upon the assumption that the life period of an excited atom varies inversely as the transition probability of the radiation emitted is presented. It is shown that recent work on the life periods of atoms in the 23S1{2}^{3}{S}_{1}23S1​ state of mercury at least partially supports this assumption. A new type of sodium arc with increased intensity and stability of operation is described.
Fluorescence has been excited in sodium vapor at 180\ifmmode^\circ\else\textdegree\fi{}C by illuminating with the second member of the principal series, $\ensuremath{\lambda}=3303\mathrm{A}$. Both the first and second members of the principal series appeared in the fluorescence, the first member being due to the atom returning to its normal state in steps. The ratio of the intensities of the two lines was determined by photographic photometry and the results compared with what might be expected on the basis of measurements by Weiss on the sodium arc. In the arc the atom returns to the normal state from the 3P3P3P by way of the 2S2S2S twentyfive times as often as the direct transition occurs whereas in fluorescence approximately the same number take each path. This is contrary to the usual assumption that the transition probabilities are independent of the method of excitation.
The intensity ratio D2/D1 for the sodium doublet has been measured for radiation from excited sodium atoms which result from the optical dissociation of sodium iodide vapor by ultraviolet light in the 2082→2500 A region. With shorter wavelength ultraviolet light as exciting source, the D2/D1 ratio is 0.98 and increases to above 1.5 with longer exciting wavelengths. Addition of foreign gases cause the observed D2/D1 ratio to increase to 1.89 for large foreign gas pressure. Cross sections for the transfer of excited sodium atoms from the 32P32 to the 32P½ state and vice versa due to collisions with foreign molecules have been measured as they depend on the relative velocity between excited sodium atom and foreign molecule. The Doppler broadening of the separate D lines emitted in the atomic fluorescence of sodium iodide has been estimated from absorption measurements. Both lines show equal broadening within the experimental error of the observations. Measurements were made with a photomultiplier tube. The sodium doublet was resolved with a reflection grating monochromator.
The behavior of the sensitized fluorescent spectrum of sodium, produced by irradiating Hg–Na–N2 mixtures with Hg 2537 Å resonance radiation, was studied in relation to N2 pressure. In this ternary vapor-gas system, the sodium atoms become excited by collisional transfer from Hg 63P1 atoms and also from Hg 63P0 atoms which are formed in Hg(63P1) + N2 collisions. The spontaneous decay of the collisionally populated S, P, and D sodium states gives rise to the fluorescent spectrum and their radiationless decay (quenching) caused by collisions with N2 molecules, manifests itself in a decrease of the fluorescent intensities as N2 pressure is increased. An analysis of the variation of the fluorescent intensities with N2 pressure, yielded 15 quenching cross sections for various S, P, and D sodium states, which appear to exhibit resonance properties with respect to upward vibrational transitions in N2.
An upper limit of ${10}^{\ensuremath{-}2}$ A˚2{\mathrm{\AA{}}}^{2}A˚2 is obtained for the quenching cross section of sodium resonance fluorescence when the excited sodium atoms are in collision with inert-gas atoms in their ground state. Excitation of sodium vapor at a pressure of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ Torr is followed by observation of the decay of the fluorescence using the method of delayed coincidences. The decay time of the fluorescence is unaffected by the presence of the inert gases He, Ne, A, Kr, and Xe at pressures up to 800 Torr, and at temperatures of 130 and 210\ifmmode^\circ\else\textdegree\fi{}C.
Effect of nitrogen and of mixtures of nitrogen and carbon dioxide upon the fluorescence of mercury vapor. By "stepwise" optical excitation, unexcited mercury atoms are raised to the 23S12^{3}S_{1}23S1​ state. They may leave this level upon the emission of radiation by four paths---the first to the 21P12^{1}P_{1}21P1​ state, a transition of relatively low probability, and the other three to the 23P22^{3}P_{2}23P2​, 23P12^{3}P_{1}23P1​, and 23P02^{3}P_{0}23P0​ states with the emission of 5461A, 4358A and 4046A, respectively. Measurements, by totally independent methods, of the mean life of the 23S12^{3}S_{1}23S1​ state yielded values, when measured by the line 5461A, equal to approximately four times those when measured by either 4358A or 4046A. Upon the addition of foreign gas, then, the intensity ratio of 5461A to either 4358A or 4046A should decrease with increasing pressure while that of 4046A to 4358A should remain constant. However, these ratios were all found to be invariant with pressure, up to 400 mm in the case of nitrogen and up to 100 mm in the case of the nitrogen-carbon dioxide mixtures.
The cross sections for the collisional transfer of excitation between the 2P3/2 and 2P1/2. levels of the first excited state of sodium have been measured using nitrogen as the buffer gas. In addition, quenching cross sections for non-radiative transitions to the ground state have been determined and compared with previous results. The experiments were carried out at sodium vapor pressures of less than 10-6 Torr so that radiation trapping is negligible. The excited states of sodium were produced by optical excitation and the nitrogen pressure range was 0 - 10 Torr.