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ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
Nanosecond Time-Resolved Spectrometry with a Tunable Dye Laser and a Simple Pulse-Gated Photon Counter Totaro Imasaka, Teiichiro Ogawa, and Nobuhiko Ishibashi" Faculty of Engineering, Kyushu University, Fukuoka 8 72, Japan
The design and construction of a fluorometric system with a nitrogen-laser-pumped dye laser ( A t = 4 ns, AA = 3.5 pm) excitation source and a gated ( A f = 4 ns) photon counter signal processor Is described. The time resolution of the system Is about 5 ns. The system can be applied to the lifetime measurement of nanosecond decays. The system was used for the measurement of time-resolved spectra and for the detection of molecular fluorescence at A$J = 5 X lo-", where A Is the absorbance and C#I the fluorescence quantum yield.
T h e development of high-power, high-resolution, and reliable lasers has accelerated the use of fluorescence spectrometry as a n analytical tool. Above all, a nitrogen-laserpumped dye laser has been a very useful excitation source for atomic ( 1 ) and molecular (2)fluorometry because of its wide tuning range. Since this laser has a very short pulse, a n excellent time-correlated detection system is essential. A boxcar integrator offers a range of variable sampling gatewidths down to 10 ns and plug-in heads with the minimum width of 100 ps. However, the system of analog detection and boxcar averaging places a severe limitation on the sensitivity. A photon counting technique can increase the sensitivity, and techniques using a sampling oscilloscope (3, 4 ) or a timeto-amplitude converter (TAC) (5, 6) have been developed to take advantage of their high sensitivity and their high time resolution. However, t h e maximum counting rates of such a system are relatively low. A pulse-gated photon counting technique has been applied to detect very weak Raman scattering (7-9); however, the time resolution of the previous apparatus did not allow the nanosecond time-resolved spectrometry. T h e design and the construction of a n apparatus for nanosecond time-resolved spectrometry with a nitrogenlaser-pumped dye laser and with a simple pulse-gated photon counter will be described in this paper. The apparatus is very sensitive and is capable of observing multiple-photons per laser pulse for long-lived photoemission. INSTRUMENT DESIGN T h e basic components of the experimental apparatus are a nitrogen-laser-pumped dye laser and fluorescence detection equipment, as shown in Figure 1. Each element will be described in detail in subsequent sections. N i t r o g e n Laser. T h e nitrogen laser was constructed on the design of a LC inversion type. It consists of a 65-cm long discharge tube, 100-nF capacitors (Nichicon, 5 n F X ZO), and a pressurized spark gap switch. T h e nitrogen laser was operated at the repetition rate of 15-25 Hz. T h e output power and the pulse width were about 100 kW and 8 ns, respectively. Forced air cooling was requisite to its stable operation. Adequate shielding against the large quantity of radio frequency interference (RFI) was very important for photon counting. D y e Laser. T h e dye laser cavity follows the design of Hansch ( I O ) , consisting of a grating, a beam expander (Oriel, X 20), the dye cell, and an output mirror. A 600 grooves/mm 0003-2700/79/0351-0502$01.OO/O
reflecting grating (30 X 30 mm) blazed for 500 nm was used in 7th order of the back of the facets (11, 12). M o n i t o r i n g C i r c u i t for Dye L a s e r O u t p u t . T h e intensity of the dye laser was monitored by a n HTV R905 photomultiplier. The dynode voltage divider was tapered for a pulsed signal (13). The signal was amplified by an operational amplifier (Teledyne Philbrick 1029); the time constant was adjusted to 4.5 s for smoothing the pulsed signals. This circuit is also useful for the measurement of the time-integrated spectrum when the fluorescence intensity is considerably strong (12). Wavelength Isolation. Photoemission was focused by a lens onto the entrance slit of either a 1-m single (JASCO CT-100) or a 0.4-m double (JASCO CT-40D) monochromator. The former was used for the high-resolution spectrometry of gaseous molecules and the latter for the measurement of very weak photoemission. The photons were detected with a fast response squirrel-cage photomultiplier (R928) having a red sensitive photocathode (185-930 nm). T h e base was wired for single photon counting according to the manufacturer's recommendations except charging capacitors which were wired to the tube socket (13). Gated Electronics. An electronic system with fast time response and high sensitivity has been constructed for the time-resolved spectrometry. The advantages of the present technique are simplicity, low cost, and easy construction. A photodiode (LSD 39A, rise time 0.35 ns) received the dye laser and triggered the gate pulse generator shown in Figure 2. The circuit consists of two monostable multivibrators made of a digital IC (SN74S00, 3.5 ns). T h e time delay and t h e gate width can be changed by adjusting capacitors and resistors. T h e widths were monitored by a synchroscope. T h e photoelectron signal was gated and amplified with a videoamplifier (MC 1445, 3.5 ns), as shown in Figure 3. A transistor was used for adjusting the characteristic impedance to 50 R. This circuit was connected just below the photomultiplier (R928) in order to minimize the RFI noise and to improve the time resolution by reducing stray capacitance. T h e signal pulse was counted by an N F PC-545A photon counter (10 MHz). The output signal was recorded either on a digital printer or on a strip chart recorder. EXPERIMENTAL The 4-methylumbelliferone (4-MU), 2-phenyl-5-(4-biphenyl)-1,3,4-oxadiazole(PBD), and aluminum Calcein Blue (Al-CB) were the laser dyes used. The fluorescence cell for a liquid sample was cylindrical, 6 cm in height and 4 cm in diameter. That for atmospheric NO2 had several iris diaphragms to reduce the scattered light (14). The fluorescein was obtained from Tokyo Kasei and it was purified from water. The NO2 was continuously generated by use of a permeation tube (Kitazawa); the air was purified by a silica gel column and a carbon column. The time-resolved spectrum was measured several times and the data were accumulated and replotted manually. Typically it took about 8 h for recording a spectrum. RESULTS AND DISCUSSION Wavelength Resolution. The line width of the dye laser is of importance for the measurement of the high-resolution excitation spectrum especially for gaseous molecules. With C 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979 GATE AHPLI
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TIME Figure 4. Raman and fluorescence intensity-time profiles. (A) 0-0-0 Raman signal of water, (B) 0-0-0 fluorescence signal of fluorescein in water (lo-' M; pH 13,NaOH). The measurement was carried out under the conditions of 0.5 count/pulse in the duration of 0-100 ns. The maximum count rate was 0.02count/pulse at 8 ns after excitation. Oxygen was not excluded from the sample. Excitation source, PBD laser; A,, = 377.1 nm, AAeX= 0.4 nm
Flgure 1. Block diagram of the experimental apparatus
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Figure 2. Circuit design of gate pulse generator. The time delay of the output signal (OUT 1) is 0 ns and the gatewidth can be adjusted to 0-1 ms. The time delay and the gatewidth of the output signal (OUT 2) can be adjusted to 40 ns-1 ms and 0-1 ms, respectively. All the measurements were carried out by using the gate pulse (OUT 1) except for the time-profile measurement
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IN 3 00 -6 V Figure 3. Circuit design of gated videoamplifier. Atthough the rise times of MC 1445 are 3.5 ns, the gate operation at the first stage in the MC 1445 and the discrimination of the low electron pulse in the photon counter seem to improve the resolution of this system
careful adjustment of the collimating telescope and the grating, a line width of 3.5 pm of full width a t half-maximum (fwhm) was obtained for the 4-MU laser. T h e resolution of the dye laser system was limited by the area of the grating (450 nm/600 grooves x 30 mm x 7th order = 0.0036 nm). When the line width was of little importance, the first-order diffraction of the grating was used for obtaining higher output power. The reproducibility of the wavelength was confirmed by measuring the high-resolution excitation spectrum of NOz (15). Time Resolution. The time resolution of the photon counting system was evaluated by observing the response of the Raman scattering of water, since it appears instantaneously. The time profile is shown in Figure 4(A). The fwhm of the pulse, hence the time resolution of the total system, was about 5 ns. The pulse width of the dye laser was measured to be 4 ns by a photodiode (0.35 ns) connected to a SS-6200 synchroscope (1.7 ns); then, the resolution of the detection system was estimated to be about 4 ns. Most of the published pulse-gated photon counting systems have not been so fast. T h e present apparatus allows nanosecond time-resolved spectrometry and nanosecond lifetime measurements. Counting Rate. T h e maximum counting rate of the present system was limited by the counting rate of the photon counter and by the repetition rate of the dye laser. A distinct
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Figure 5. Fluorescence spectrum of atmospheric NO,( 100 ppm). Fluorescence was monitored in the duration of 0-300 ns. Excitation source, AI-CB laser, A,, = 424.3 nm, LAex= 0.25nm; LAe,,, = 5 nm. The ordinate shows the number of photoelectrons per 80 s
advantage of this system is that for long-lived emitters i t is possible to detect multiple-photons per laser pulse. When the present circuitry is combined with a low repetition rate N2 laser, the counting rate is similar to that of a typical TAC system for fluorophors of very short lifetimes (
