in the structure that does not appreciably interfere with the transmission of the uv photons or photoelectrons or the operation of the ion source in the ims in fig2 c an electrically heated heater tape 40 is shown as being used to heat the photocathode 26 the heater tape 40 may be disposed at or near the surface of the photocathode 26 the heater tape 40 may be an electrically conductive coating on the optically transparent window 31 the heater tape 40 may be disposed on the substrate of the photocathode 26 or on the photocathode 26 itself the photocathode 26 and or the substrate thereof may be embedded in an electrically insulating material to avoid shorting out the heating element within the heater tape 40 may be fabricated from a variety of materials such as polyimide foil or silicone the heater tape 40 and corresponding electrical leads 29 may be clamped or glued to the substrate of the photocathode 26 and or the optically transparent window 31 the temperature of the photocathode 26 and or the substrate of the photocathode 26 may optionally be monitored using a temperature sensing device 27 mounted thereon the temperature sensing device 27 may provide data to the independent temperature control system 13 not shown in fig2 c the temperature sensing device 27 may be an rtd a thermocouple a thermistor or any other appropriate device or combination of devices all of the materials of these several components may be selected.