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 Px give deflections proportional to the voltage on the Kerr cell K. C. while the plates Py give deflections proportional to the voltage on C2 and therefore in phase quadrature with the current through the discharge tube. L1C1 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. L2C2 forms a series tuned circuit of low impedance, and the variable condenser C7 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.
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 Px give deflections proportional to the voltage on the Kerr cell K. C. while the plates Py give deflections proportional to the voltage on C2 and therefore in phase quadrature with the current through the discharge tube. L1C1 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. L2C2 forms a series tuned circuit of low impedance, and the variable condenser C7 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.
The mean life of the mercury line $\ensuremath{\lambda}2537$ was measured by the alternating voltage method previously described by Webb. The radiation was excited in a quartz tube by impact with electrons from a hot cathode. The photoelectric system was contained in a separate tube also of quartz. The photoelectric surface was zinc evaporated on a nickel plate. Measurements of the apparent life were made with diminishing mercury vapor pressures until further reduction of pressure produced no change in the measured rate of decay of the radiation. At these low pressures (below 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ mm, for the geometry used) the absorption and reemission of the radiation was negligible and the true life of a single excitation process was measured. This was found to be 1.08\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ sec. with an estimated precision of one percent.Absorption coefficient of $\ensuremath{\lambda}1849$. An absorption cell with a one mm thick absorbing layer of mercury vapor was mounted so as to move in and out between the excitation and photoelectric tubes used in the above measurements. The absorption of $\ensuremath{\lambda}1849$ was measured at pressures such that the absorption of $\ensuremath{\lambda}2537$ was negligible. From this the atomic absorption coefficient for the main component of the line was found to be 6.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ from which the mean life of the 21P1 state was calculated to be 2.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec. with an estimated precision of 25 percent.
Abstract When a beam of Canal-Rays passes into a high vacuum, it is well known that the light-intensity falls exponentially to a constant value which, if the pressure is sufficiently low, is very small indeed. The experiments of Wien, and the theory developed by him, showed that the observations could be equally well accounted for on either of two hypotheses, namely that the light from each atom falls off exponentially at the observed rate, or alternatively, that all the energy from any one atom is emitted in a very short time indeed, and that what is measured is essentially the decay-curve of those atoms that are in the initial state for the radiation in question. As to whether the maximum emission (on either theory) occurred exactly at the point of excitation, however, these experiments provided very little evidence; the excitation was in fact, so far as can be determined, distributed continuously over a region about which all that can be said is that it was bounded more or less definitely on one side by the back of the cathode, and it was possible that the curves for a point-excitation would really resemble, for instance, the activity-curve of an intermediate radioactive substance initially absent from a preparation. In fact plausible grounds for believing that this should be the true form were ready to hand, since the atom might very well be expected to spend some time in radiating infra-red lines before it reached the initial state for the radiation observed; on this point all that could be inferred from Wien’s curves was that the interval must at most be appreciably smaller than the half-period of the exponential fall. This interval is not the same as the “Yerweilzeit” considered by him; the Verweilzeit is the basis of a demonstration that the curves due to point-excitation should be exponential, while the present interval would have the effect that they should not. In order to decide whether there was in fact any delay of this nature at all, it seemed desirable to try whether non-luminous canal-rays, some distance after their entry into the vacuum, could be excited anew to luminescence at approximately one known point only, so that any interval between excitation and emission could be directly measured. The experiments devoted to this object have been somewhat interrupted, and it is further necessary to discontinue them, at least for a time, once more. The present paper is thus an interim account, containing a description of the method that has been worked out and the result that has been obtained; there are, however, a number of further questions which it was hoped to decide, and which presumably can be decided by the same method, that are still unsettled.
Broadening of the satellites of the Hg green line $\ensuremath{\lambda}5461$ was observed when the light passed between the plates of a Kerr cell on which was impressed a varying E.M.F. having a frequency of 2\ifmmode\times\else\texttimes\fi{}107 ${\mathrm{sec}}^{\ensuremath{-}1}$. The E.M.F. was obtained by superposing the output of a vacuum tube oscillator upon a steady potential of about 7000 volts. The maximum oscillator voltage was about 5000 volts. With this voltage the fine structure of the Hg green line became so diffuse that two satellites having a separation of 0.045A were scarcely resolved by the Lummer-plate. The broadening was observed to increase rapidly with the oscillator voltage. The observed effect is predicted by the classical electromagnetic theory and the quantum theories of dispersion of Kramers-Heisenburg, Schr\"odinger, and Dirac. The high frequency voltage effects a rapid variation of the refractive index of the nitrobenzene in the Kerr cell which in turn produces a corresponding variation in optical path, giving the source a virtual velocity, and thereby producing a Doppler effect. The magnitude of the observed boradening is in agreement with the predictions of theory.
The mean life of the 2S state of hydrogen in the absence of collisions is primarily determined by the simultaneous emission of two photons. Owing to this cause the mean life is `-*4 sec. It is expected that the mean life of is 2S `S of He i is of the same order of magnitude and is much shorter than that of is 2S 3S of He i. The non-adiabatic collisions with electrons are found to be more important than the static effects at small electron densities, leading to a transition probability `-*-`~h sec ` for an electron temper- ature `-.--`io,ooo° C and an electron density 30 cm3. The effect of static electric fields on the 2S state of hydrogen is worked out in more detail than previously by taking into account the hyperfine structure. The mean life in static electric fields which are strong enough to make double emission of little importance and yet weak enough to give a mean life > io7 sec is found to be the sum of two lives: (i) the mean life calculated neglecting hyperfine structure and (2) the mean life calculated neglecting radiation coupling. In both (i) and (2) the mixing of 2~ into 2S by the Stark effect is taken into account. Radiations due to quadrupole and magnetic dipole effects are considered qualita- tively for is 2S 3S of helium and quantitatively for 2S of hydrogen. For the latter there exists a small radiative magnetic dipole effect on Dirac's theory of the electron which is absent in the ordinary spin model
The magnetic moment of the hydrogen atom has been investigated by the atomic ray method introduced by Stern and Gerlach. Atomic hydrogen formed in a discharge tube by the method of R. W. Wood was first used. The ray was formed in a special all-glass slit system of three slits sealed to the discharge tube. The ray was detected by the reduction resulting on contact with a target coated with molybdenum trioxide. A sharply defined blue line against a white background was the result. In the magnetic field the ray was separated into two branching rays. There was also evidence of a central underviated ray which is believed to be due to hydrogen active chemically but probably not in the atomic state. From a measurement of the deflection the magnetic moment of the hydrogen atom was calculated to be one Bohr magneton within the limits of experimental error. This result is of interest because of the questions raised by the new quantum mechanics of Heisenberg, Born, and Jordan, and by the spinning electron theory of Uhlenbeck and Goudsmit. Atomic hydrogen formed by the hot filament method of Langmuir was next used. The increased velocity of the atoms in this case resulted in less separation of the ray, but a deflection was distinctly recorded. Finally, the product formed on exposing a mixture of mercury vapor and hydrogen to ultra-violet light was investigated. This is believed by Cario and Franck, Taylor, and others to be atomic hydrogen. Thus far attempts to form a ray which will reduce the target have been unsuccessful. The reasons for this are being investigated.
Values of rate of decay and mean life of atoms and molecules in upper quantum states calculated from data on the intensity of absorption lines.---By combining F\"uchtbauer's method of determining from the intensity of absorption lines the probability that a molecule will absorb a quantum of energy, with Einstein's views as to the mechanism of light absorption and emission, the following equation is derived for calculating the rate at which molecules jump from upper to lower quantum states: ${A}_{21}=\left(\frac{8\ensuremath{\pi}{\ensuremath{\nu}}^{2}}{{c}^{2}{N}_{1}}\right)\left(\frac{{p}_{1}}{{p}_{2}}\right)\ensuremath{\int}{0}^{\ensuremath{\infty}}\ensuremath{\alpha}d\ensuremath{\nu}$ where A21 is the chance per unit time that a molecule will jump spontaneously from quantum state 2 to quantum state 1, $\ensuremath{\nu}$ is the frequency of the light emitted in such a jump, p1 and p2 are the a priori probabilities of quantum states 1 and 2, and $\ensuremath{\alpha}$ is the absorption coefficient of the substance measured under conditions such that N1 is the number of molecules per unit volume in the lower quantum state 1. The integral $\ensuremath{\int}\ensuremath{\alpha}d\ensuremath{\nu}$ is to be taken over the total effective width of the absorption line corresponding to the passage of molecules from quantum state 1 to quantum state 2. The mean life $\ensuremath{\tau}$ of molecules which decay from state 2 to state 1 is the reciprocal of A21. Values of A21 and $\ensuremath{\tau}$ are calculated from existing data for the mercury line $\ensuremath{\lambda}2537$, for a number of lines belonging to the alkali doublets, for the iodine line $\ensuremath{\lambda}5461$, and for a very considerable number of lines belonging to the rotation-oscillation spectra of the hydrogen halides. The values obtained agree with the meager data made available by other experimental methods. From these results the following conclusions are drawn.The mean life of molecules and atoms in upper quantum states may vary for different states at least over the range 1 to ${10}^{\ensuremath{-}8}$ seconds. The rate of decay is not a simple function of the frequency of the emitted light. The rate corresponding to the emission of a line of high frequency may be greater or less than that for a line of lower frequency. The data now available for the alkali doublets $1s\ensuremath{-}m{p}_{1}$ and $1s\ensuremath{-}m{p}_{2}$ indicate a higher rate of decay the smaller the change in total quantum number for the line under consideration. The rate of decay from a given mp1 state is m times as great as from the corresponding mp2 state (already stated in another form by F\"uchtbauer and Hofmann). In the case of the rotation-oscillation spectra of the hydrogen halides the rate of decay is greater for quantum states with one unit of oscillation and many of rotation, than for those with one unit of oscillation and only a few of rotation; and in the case of different molecules but the same quantum numbers the rate of decay is greater for the molecule with the greater frequency of oscillation.Finally the possibility and method of calculating absolute values of A21 from the Bohr correspondence principle is indicated.