Instrumentation
OPTICAL SIGNALS: DETECTORS F. Ε. Lytle Department of Chemistry Purdue University Lafayette, Ind. 47907
Recent awareness of the potential contributions of time-resolved spec troscopic measurements to b o t h fun damental excited-state processes (1, 2) and analytical methodology (3, 4), coupled with advances in the ability to generate short-duration light puls es (5), has led to an explosive in crease in the number of studies re ported in the literature. These experi ments are singular in t h a t the obser vation periods used encompass an al most uniquely broad span. For classi cal molecular spectroscopy, the short end of this range is limited to about 1 femtosecond (Table I) because the energy uncertainty related to t h e event becomes larger t h a n t h a t of a typical bond (6). From such a lower limit, a continuum observation period exists u p to the h u n d r e d s of seconds required with some phosphorescence. For the purpose of the present discus sion, this entire span can be subdi vided into three sections on the sole criterion of m e a s u r e m e n t technology. For moderately fast signals ( >100 nsec), available instrumentation is so common as to render its discussion blasé. Likewise, ultrafast signals ( *·— V)
c 10-ΜΩ load is n o t u n c o m m o n . T o c o m p e n s a t e for this loss of resistance, t h e anode cur rent is usually increased by allowing m u c h more light to irradiate t h e p h o t o c a t h o d e . W h e n t h i s is done, t h e m a g n i t u d e of the d y n o d e current a n d t h e m a x i m u m average anode c u r r e n t for t h e p a r t i c u l a r t u b e in consider ation are t h e limiting factors. T h a t is, t h e p e a k anode c u r r e n t still c a n n o t exceed % 0 t h a t in t h e voltage divider, a n d t h e average c u r r e n t cannot ex ceed t h a t specified by t h e m a n u f a c t u r e r . E x a m p l e 2 shows how these two restrictions c a n control t h e m a x i m u m values of t h e signal characteris tics. Example 2. Peak Signal and Repetition Rate Calculations Without Charging Capacitors Consider the experiment described in Example 1. RCA suggests that the C31025C have a voltage divider wired ex actly like the 931A, i.e., 10 equal-valued resistors. If each of them were 1 W-20 kfi, the total dynode current would be 5 mA at an applied potential of 1 kV. Thus, with a 50-Ω load resistor, the peak voltage should be kept
