The mean lives of excited hydrogen atoms

Abstract

A method of determining directly the mean lives of excited atoms was described in detail in a previous paper. This method employs a Kerr cell operated by a high frequency oscillator which is also used to run a Geissler tube. With the electrical arrangement used, a minimum of light should pass through the Kerr cell system if light were emitted instantaneously. Owing to the finite time that the atoms remain in the excited state, there is a lag between the current in the discharge tube and the emission of light. By increasing the distance that the light travels before reaching the cell a point may be found where a maximum of light passes the cell, from which the lag may be determined. This is directly connected with the mean life of the state. In the previous paper results for neon were given and the present paper deals with the application of this method to hydrogen. Fig. 1 shows the electrical arrangement used. This is essentially the same as that used for neon, except that a cathode ray oscillograph is used to determine the phase difference between the potential on the Kerr cell and the current in the discharge tube. Thus the deflector plates P x give deflections proportional to the voltage on the Kerr cell K. C. while the plates P y give deflections proportional to the voltage on C 2 and therefore in phase quadrature with the current through the discharge tube. L 1 C 1 is a tuned circuit which picks up energy from the oscillator and by tuning this, the oscillograph figure may be made a straight line. When this is so the current through the discharge tube is 90° out of phase with the voltage on the Kerr cell. L 2 C 2 forms a series tuned circuit of low impedance, and the variable condenser C 7 gives a means of making the two deflections equal. The oscillograph can be set for linearity within an accuracy of ½° which is of the same order as the accuracy of the experiment. A correction has to be applied for the time that the electrons take to travel between the plates of the oscillograph. Thus when the same oscillating voltage is applied to the two pairs of plates, the oscillograph figure is not a straight line, but shows an apparent phase difference, owing to the fact that one pair of plates acts on the electron beam sooner than the other pair. With the oscillograph used, this time was 1⋅5 . 10 -9 secs, or at a wave-length of 60 metres a phase angle of 2⋅7°. This must be added to the observed lag.