PHOTOEMISSION MEASUREMENTS OF TEMPERATURE IN PULSED LASER HEATING OF VARIOUS MATERIALS

The possibilities for the photoemission method of measuring the temperature of various materials heated by millisecond laser pulses have been investigated. The temperature of graphite, tungsten, tantalum, silicon plates, and silicon dioxide films was determined experimentally with a time resolution of 1 μs within the range 1200-2900 K. Introduction. Measurement of the temperature of fast processes is a complex problem in itself, and it is fur- ther aggravated if the temperature of an object changes from room to 3000 K at a rate of 10 8 K ⁄ s and there is no possibility of taking account of the accompanying change in the emissivity. In this case, to measure the temperature of the object, use can be made of the photoemission method in which modulation with a high-frequency electronic flux rather than a low one is produced, and one can compare the energy distribution of photoelectrons in two broad spec- tral intervals, one of which being part of the other, with the difference between the measured temperature and the ther- modynamic one turning out to be insignificant. The procedural error of such measurements does not exceed 0.3% at a surface emissivity approximately equal to 0.4. This property was revealed experimentally (1) and later confirmed theo- retically (2, 3). Measurement of the temperature of an object by the photoemission method from the spectra of photoelec- trons released by thermal radiation is based on the dependence of the energy distribution of photoelectrons of the ex- ternal photoeffect on the energy distribution in the object's radiation spectrum. In this case the gas of photoelectrons near the emitting body surface is the thermometric substance, and its temperature can be determined from the cut-off voltage of the volt-ampere characteristics of a photoelectronic device, for example, a photomultiplier for light fluxes normalized by the photocurrent (1) or by the change in the energy distribution of photoelectrons as a function of temperature (2, 3). As the body temperature increases, the maximum of the spectral energy distribution of its radiation is dis- placed to the side of short wavelengths (the Wien displacement), with the maximum of energy distribution of photo- electrons being displaced to the side of high energies. In this case, the body temperature is determined by the parameter k, which is the ratio of the photocurrent I 0 measured in the absence of the retarding field to the partially blocked photocurrent I bl measured in the presence of the retarding field in the cathode region of the photomultiplier. Since the relative content of electrons of different energies in this region is independent of the magnitude of the light flux arriving at the device, within the boundaries of linearity of its light characteristic k = f(T) (3). The process of measuring the body temperature is reduced to the recording of a two-level oscillogram of the amplitude of the voltage pulse at the photomultiplier anode U(t) in the process of electron flux modulation in the cath- ode region of the photomultiplier by rectangular pulses that are negative relative to the photocathode. Using these lev- els of the photomultiplier signal, the function k(T) is calculated in each period of the modulated signal, and using the calibrating function T(k) obtainable with the use of the standard thermal radiation, one determines the dynamics of the temperature of the investigated body T(t). The simplicity of the electron flow modulation in the cathode region of the photomultiplier with a frequency of 1 MHz allows one to make such measurement with a time resolution of 1 μs. The possibilities of the indicated method in measuring the temperature of fast thermal processes is illustrated below using several examples. Using an FMK-01 pulse photoemission pyrometer, we measured the temperature of sev-