Apparatus for Time- and Wavelength-Resolved Spectroscopy

NASA's Jet Propulsion Laboratory, Pasadena, California

An apparatus that includes a pulsed electron gun and a high-resolution ultraviolet spectrometer has been developed for use in (1) pulsed electron-impact excitation of transitions among the electron-energy states in a gas and (2) measurement of the spectrum of the resulting ultraviolet light emitted by the gas, as a function of wavelength and of time after turn-off of the excitation. The apparatus is designed especially for measuring the Lyman-band emission spectrum of H₂, in order to determine the cascade contribution to this spectrum and thereby to contribute to the understanding of cascade contributions to electron-excited spectra of gases in general.

In the electron gun, electrons are generated by a thoriated tungsten filament and electrostatically accelerated through a collimating magnetic field. The electron beam collides at a right angle with a beam of H₂or another gas of interest effusing through a hole. Light emitted by the electron-excited gas is dispersed by the spectrometer, and the spectrally dispersed photons are detected by use of a channel electron multiplier coated with CsI.

Photon-Emission Rates for directly excited and cascade states decay at different rates when the electron beam is turned off. One can use this difference to discriminate against the faster decay of the directly excited states to obtain the cascade spectrum.

A version of the apparatus as described thus far has been used in prior research to excite and measure spectra in the steady state. The present version of the apparatus is distinguished by its capability for pulsed excitation and time-resolved spectral measurement. In the present version, the basic mode of operation, in which the spectrum is measured as a function of time after turn-off of the excitation, is dictated by the following consideration: Directly excited states that decay to the ground state via resonance transitions typically have lifetimes much shorter than those of cascade states; on the basis of this characteristic, it is possible to discriminate against or suppress contributions of transitions between directly excited resonance states and ground, in order to obtain the desired cascade spectrum.

The electron beam is pulsed on and off (see figure) by applying a rectangular potential waveform to one of the accelerating electrodes in the electron gun. Starting at the beginning of the pulse cycle (time t₀), the electron-beam current increases rapidly from zero to a stable "on" value (typically to ≈200 μA within a time of 150 ns). The gun is maintained in the "on" state for a time sufficient to obtain dynamic equilibrium between excitation and de-excitation processes of the specific excited state that one seeks to analyze. At time t₁, the electron beam is switched off and the electron-beam current falls to zero within ≈30 ns.

After a short gate-delay interval that ends at time t₂, the photon detector in the spectrometer is turned on by use of another rectangular potential waveform. The photon detector is subsequently turned off at t₃, before the beginning of the next pulse cycle at t₄. The gate-delay time (t₂ – t₁) can be made long enough that, to a first approximation, the directly excited states can be regarded as having decayed completely to ground. The pulse-repetition period (t₄ – t₀), the "beam on" time (t₁ – t₀), and the gate-delay time can be varied over a wide range, as needed for analyzing states characterized by effective lifetimes from tens of nanoseconds to≈8 μs.