A direct lifetime measurement for a resonance transition in argon Public Deposited

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  • The 4s³P₁ state of argon decays radiatively by a resonance transition to the 3p⁶¹S₀ ground state; the wavelength of this radiation is 1067A. By direct observation of the decay of the 1067A radiation from excited argon gas, a value of 0.93±0.06 x 10⁻⁵ seconds has been obtained for the natural lifetime of the 4s³P₁ state. The failure of this state to appear as a metastable state points up the inadequacy of a description of the argon atom by Russel-Saunders coupling. The calculated value (25) of 1.05 x 10⁻⁵ seconds for the natural lifetime of the 4s³P₁ state requires a larger degree of mixing of the strict Russel-Saunders terms than does the value measured in this experiment. The measurement was made using a modified delayed coincidence method (3). Bombardment of argon gas with a pulsed beam of controlled energy electrons was used to produce cyclic excitation and relaxation of the gas. The cyclic regeneration of the argon decay allowed the entire decay curve to be determined by examining it piecemeal. A small portion of the decay curve is repetetively examined by gating a detector such that it is active only during a small fraction of each cycle. During each cycle the location in time of this sampling interval is set by triggering the gating signal with a delayed pulse which is produced at the termination of the excitation. Different portions of the decay curve are examined by adjusting the delay of the trigger pulse. Because the detector output is a constant which varies only when the location of the sampling interval is shifted along the decay curve, this method avoids the problem of attempting to record a single transient. The atomic excitation is produced within a nine liter cylindrical volume. A slow molecular type flow of argon is maintained through this excitation chamber. The pressure is essentially uniform within this chamber and is varied over the range of 10⁻³ to 10⁻¹ Torr. The electron gun, located at the center of the excitation chamber, consists of the cathode and grid structure of a 6SJ7 electron tube. The spread in energy of electrons from this gun was small enough to allow selective excitation of the 4s multiplet of argon, but selective excitation of levels within this multiplet could not be achieved. Radiation from the excitation chamber was detected with a Bendix magnetic photomultiplier (20) which was separated from the excitation chamber by a thinly cleaved lithium fluoride window. This photomultiplier is sensitive only to radiation in the wavelength region between 2A and 1500A. Final identification of the observed radiation as that from the 4s³P₁-3p₆¹S₀ transition was made using a vacuum spectrograph. Since the intensity of radiation from the excitation chamber is directly proportional to the concentration of argon atoms in the 4s³P₁ state, it is the decay of these atoms that is determined. This decay will be governed by the radiative decay constant only if collisional transfer of excitation and trapping of resonance radiation can be neglected. Throughout the range of pressure used in this experiment the rate constants associated with both these processes are known to be pressure dependent (21, 22) and the effect of resonance trapping can be observed separately by its dependence upon the enclosure geometry. It is to be expected that the predominant collision process will be the two body collisions which result in the exchange of excitation between the 4s³P₁ level and the adjacent metastable levels. The experimental data consistently yield decay curves which represent the sum of two exponential decays having different decay constants. The larger of these decay constants is independent of gas pressure and the smaller one is approximately directly proportional to pressure. Neither is dependent upon enclosure geometry. Therefore, it is concluded that the effects of resonance trapping are negligible, and that the de-excitation of argon atoms in the 4s³P₁ state is governed predominantly by radiative decay and by two body collisions which result in the transfer of excitation to or from this state. The cross sections for collisional transfer is approximately 4 x 10⁻¹⁵cm² at room temperature. The fact that resonance radiation from the 4s¹P₁ state of argon was not observed indicates that this radiation is heavily trapped and that collisional transfer is primarily responsible for the decay of this state. It thus appears that there is a large difference in the natural lifetimes of the two radiative 4s levels. This implication is in disagreement with the experimental results obtained by Vaughan & Stacy (35).
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