J. N. Demas, R. P. McBride, and E. W. Harris
2248
Laser Intensity Measurements by Chemical Actinometry. A Photooxygenation Actinometer J. N. Demas," R. P. McBride, and E. W. Harris Department of Chemistry, University of Virginia, Charlottesville, Virginia 2290 1 Received June 25, 1976)
(Received January 26, 1976: Revised Manuscript
Publication costs assisted by the Petroleum Research Fund
A new chemical actinometer especially designed for intensity measurements on high power lasers in the uv to green region is described. The actinometer consists of a closed 02-filled system containing methanolic tris(2,Z'bipyridine)ruthenium(II) chloride ([Ru(bpy),]Clz) and tetramethylethylene (TME). The highly colored Ru(bpy)gZ+ absorbs the laser radiation to form a long-lived charge-transfer excited state which is efficiently quenched by dissolved 0 2 to form singlet oxygen ( l 0 2 ) . The lo2 reacts efficiently with TME to give a nonvolatile hydroperoxide. The laser intensity is determined from the rate of 0 2 consumption monitored on a gas buret. The system is easy to use, has a quantum flat response over the 280-560-nm region, and is particularly suited for the ionized Ar laser lines. Detailed characterization and use of the system are presented.
Introduction Intensities of high power lasers are commonly measured with calibrated thermal detectors and silicon ph0todiodes.l-8 This procedure has a number of difficulties, however. Although home-made power meters of great accuracy can be constructed, they are expensive and difficult to use. Consequently, most workers use commercial meters with factory calibrations or one of the few commercial ones having their own internal heaters for calibration and restandardization. Clearly, most results are subject to the manufacturer's competence, but drift or detector degradation can destroy the best original calibration. Even most self-calibrating thermal detectors are subject to undetectable drift from target degradation. Further, most detectors have small apertures and are unusable with the large beam of many photochemical and spectroscopy experiments. Scientech makes 1- and 4-in. diameter units, but the 4-in. unit in particular is quite expensive. Most manufacturers will recalibrate their power meters, but this is unsatisfactory when time, cost, and the need for frequent standardization are considerations. Chemical actinometry overcomes many of these problems and supplies a complementary tool for power measurements. An actinometer is a chemical system which undergoes an irreversible photochemical reaction. Once the efficiency of the photoreaction is calibrated using an absolute method, the actinometer can be used as a secondary standard for measuring absolute light intensities from the rate of the lightinduced reaction. Actinometers can have the following advantages: (a) They are inexpensive and once calibrated by an absolute method can be used for absolute calibration of all types of thermal or photoelectric detector. They are the only power meters which can be mailed on a postcard. (b) Since the chemical reaction is always reproducible,, actinometers are ideal for monitoring and correcting long term drift in other detectors. (c) Results referenced either directly or indirectly to the actinometer can always be corrected if the absolute yield for the actinometer is ever revised, while results referenced to an improperly calibrated meter would rarely be recoverable. The Journal of Physical Chemistry, Vol. 80.No. 20, 1976
(d) Some actinometers will detect the quantum content of the beam, independent of its spectral distribution (quantum flat). To obtain the quantum content of a multiline laser with a thermal detector, the laser's spectral distribution must be accurately known and this is a frequently difficult task. Further, thermal measurements are notoriously sensitive to errors from infrared, while actinometers can be tailored to detect some wavelengths and discriminate against others, including infrared. (e) Actinometers can be made easily in virtually any physical size and shape, including nearly completely surrounding a sample to intercept almost 4n steradians of radiation. While no previously existing actinometer is completely satisfactory for high power laser^,^ we report a new singletoxygen actinometer which overcomes most of the previous shortcomings, gives exceedingly good reproducibility, uses commercially available chemicals, is easy to use, and requires no sophisticated equipment. It is also usable to wavelengths less than or equal to the 514.5-nm Ar line, to CW powers of 21.5 W, and is almost certainly quantum flat in response over the 280-560-nm region.
Actinometer The basic system we have adopted is the tris(2,2'-bipyridine)ruthenium( 11) chloride ([Ru(bpy)a]Cl2) sensitized photooxidation of tetramethylethylene (TME) in methanol. The physical and chemical processes are
hu
[ R U ( ~ P Y ) Q ] ~*[Ru(bpy)3I2+ +
(1)
@,sc
*[Ru(bpy)3I2++ 0
k,
2
[ R ~ ( b p y ) ~+] 0~ 2+t heat
kioz 102
0 2
+ heat
(4) (5)
2249
Laser Intensity Measurements by Chemical Actinometry
lo2
+ TME
-
kqTME
+
O2 TME
END V I E W
+ heat
(7)
where *Ru(bpy)S2+ is the complex in its long-lived excited state, @iSc is the efficiency of production of this state following optical excitation, 0 2 is ground state oxygen, and lo2 is metastable excited singlet 0 2 . The rate constant for each process is indicated over the arrow. Deactivation of * R ~ ( b p y ) 3 ~ by+ TME has been omitted since it is an unimportant process.lOJ1 This kinetic scheme gives the following instantaneous observed yield, 4obsd, for disappearance of 02 (mol of 0 2 consumed/einstein of photons absorbed; 1einstein = 6.023 X loz3 photons):
I
r + ''
100
40 = @isc4rx4et 4rx
= krx/(krx
+ kqTME)
P = k1'2/krx
Ks,
4et
= ket/(ket
= (ket
+ kq)
+ kq)/kl
where 4rxand 4 e t are the probabilities of reaction of lo2 with T M E a t infinite TME concentration and of energy transfer from *Ru(bpy)S2+to 0 2 at infinite 0 2 concentration, respectively. [ 0 2 ] and [TME] denote molar concentrations of 0 2 and TME, respectively. In practice 4obsd can not be calculated directly from eq 8, because both [TME] and [Oz] change during photolysis. The procedure for obtaining 40 from measured quantities will be described in the Experimental Section. Ru(bpy)Sz+ luminesces strongly in methanol, and K s , is easily obtained from the variation of emission intensity, 8, with [ 0 2 ] in the absence of TME from
( ~ O l O- 1) = Ks,[Ozl
(9)
where the subscript 0 denotes the value in the absence of 02.12
0, while not readily obtained by our experiments, has been measured by the variation of &bsd with [TME] at very low TME concentration^.^^ In all calculations, we use Foote's value of 0.0027 M.13 The actinometer consists of a system with a gas buret filled with pure 0 2 connected to the photolysis cell (Figure 1). A methanolic solution of [Ru(bpy)3I2+and T M E forms the actinometer. The strongly orange-colored [Ru(bpy)3I2+absorbs the radiation, and the excited complex efficiently generates lo2 by collisional deactivation with 0 2 . lo2 is then efficiently consumed by TME to yield a nonvolatile hydr0pero~ide.l~ After irradiation and reequilibration, the degree of reaction ( 0 2 uptake) is read directly from the gas buret. The quantum dose absorbed by the actinometer, Iabs(einstein), is given by Iabs
= nOz/$'obsd = PatmV02(m)/RT&obsd
(10)
where no2 is the number of moles of 0 2 absorbed, VO,(CO) is the total volume of pure 0 2 consumed, Pat, is the atmospheric pressure, R is the gas constant, T(K) is the temperature of the apparatus, and @obsd is the effective quantum yield for the reaction over a finite irradiation period (moles consumed/ einstein). I a b s differs from the product of irradiation time and free air flux, lO(einstein)/s, because of the transmittance, T,, of each actinometer cell window (reflection losses) and because a fraction of the beam, Ti, passes through the sample on a single pass without being absorbed. Io is given in terms of observables by
-I
Figure 1. Photolysis apparatus: (A) Coherent Radiation Laboratories Model CR-5 laser; (B) 8-cm focal length Pyrex lens; (C) power meter detector head; (D)magnetic stirrer; (E) Pyrex photolysis cell (7.6 cm diameter windows and 5 c m pathlength) with Teflon stirring bar. End view shown in insert. The filling stem must be big enough to accept the stirring bar. Constant temperature water (0.1 "C) is circulated through the jacket; (F) capillary tube 8 c m X 1 mm i.d. inserted in a rubber stopper; (G) Teflon stopcock; (H) Tygon tubing; (I) 100-ml inverted liquid buret; (J) 250-ml separatory funnel used as leveling bulb. The leveling fluid was ethylene glycol. All distances (cm) are approximate.
where tirr is the irradiation time, and r is the effective reflectance of the glass-air-solution interfaces to the beam. To use the actinometer for measuring Io's, one must know T,, T,, and 4/obsd. Since T , and T, are readily measured or calculated from Fresnel's laws, calibration of the actinometer consists of obtaining 4'obsd by an absolute method. Since, as we shall show, @obsd is easily calculated from observables and 40, calibration consists of determining 40.
Experimental Section Materials. [Ru(bpy)3]Clz.GH20 was obtained from G. Frederick Smith Chemical Co. and used without purification or after a single recrystallization from water. TME was either from Aldrich (99+% Gold Label) or an old bottle from Chemical Supply Co. (99%); both dissolved completely in methanol to give a clear solution and were used without further purification. Methanol was AR grade and was used without purification. Spectrofluorimeter. The spectrofluorimeter described elsewhere10J4 was used for all measurements. Absolute Power Meter. Absolute intensities were measured with a Jennings-West absolute b ~ l o m e t e r .The ~ -38-mm diameter solid aluminum target was equipped with a 6 0 4 Manganin calibration heater and a four-junction copperconstantan thermopile. The target was blackened with Krylon 1602 flat black spray paint by applying several light coats. The thermopile readout was a Keithley Model 155 microvolt null detector. Photolysis Apparatus. The complete photolysis setup and cell are shown in Figure 1.Such a cell is readily fabricated by a competent glass blower. Alternatively, an unjacketed cell could be mounted in a thermostated block. The stopcock and capillary tube minimized diffusion of methanol into the O2 reservoir. All gas interconnections and stopcocks were lubricated with silicone vacuum grease to eliminate leakage. The defocussing lens reduced power density on the actinometer and minimized reciprocity failure due to local hole The Journal of Physical Chemistry, Vol. 80,No. 20, 1976
J. N. Demas, R. P. McBride, and E. W. Harris
2250
burning in the 0 2 con~entration.~ The use of solvent free 0 2 in the gas line and the stopcock and capillary of Figure 1 is important. Considerable data were originally collected using methanol saturated 0 2 in the cell and gas line, a very reproducible system. These results were, however, -10% too low because of adsorption of methanol on the Tygon tubing. To show yields were independent of the gas composition, several methanol saturated 0 2 runs were made with the Tygon replaced by glass tubing with only very short interconnections being made with Tygon; good agreement between the two procedures was then obtained (vida infra).
Procedures and Calculations Determination of KsU,Techniques described elsewhere were used.ll Saturation 0 2 concentrations, [ 0 2 I s a t , were calculated from an Ostwald coefficient of 0.2471 at 21 O C . I 5 [02lsat under our conditions is given by [02]sat =
It is assumed that the combined air-glass and glass-methanol reflectances of a single window act like a single interface with a composite reflectance r, that there is no solvent or window absorption, and that all of the infinite interreflections exiting from the rear face strike the detector. Beam divergence makes the last assumption slightly incorrect; however, simpler equations result and the rapid attenuation of interreflected light makes the errors negligible (ca. 50.1%).The slight beam divergence should also .minimize interference effects. T, is given by
The same assumptions are made in the derivation of T, as for
Tw. Determination of 40. $0 can then be calculated from
(0.010 22 M)Po,/760
Poz = Patrn- P C H ~ O=HPatrn- exp[20.267 - 4609.0/T] (12)
Poz is the partial pressure of 0 2 , Patm is atmosphere pressure, and P C H ~ OisHthe vapor pressure of methanol16 in the photolysis cell; all pressures are in Torr. Power Meter Calibration. The electrical sensitivity, Selec (microvolt output per watt of electrical heater power), determined by passing known currents through the Manganin heater, was slightly power dependent but stable during -1 year, and all calibrations were combined. The function Seiec= (286 fiV/W) - (0.012 W-l)E
(13)
where E is the thermopile output voltage gave a good fit with an experimental scatter of -1-2%. Optical sensitivity was -1% less for a 25-mm diameter beam than for a 10 mm one. The reflectance of the target’s flat black paint, R’, was determined by comparison with a freshly smoked magnesium oxide surface with the same orientation. MgO is an almost perfect diffuse reflector,ls and we have used unity for its reflectance in our calculations. Scattered radiation was monitored with a UDT PIN-1ODP silicon photocell and a Keithley Model 160 microvoltmeter used in the current mode. The photocell was operated at 1 3 KA to ensure linear operation. Absolute laser beam intensities were given by
where X is the laser beam wavelength (nm). Evaluation of r, T,,,and T,. The reflection coefficient of the cell windows was obtained from the apparent solvent filled cell transmission. With the power meter at position no. 2 (Figure I), we measured the bolometer output with the cell in place filled to its working volume with pure methanol, Ef, and with the cell removed, E,. The apparent transmission is (EdEJsoiVent.
The d.ose absorbed by the actinometer was obtained from the apparent actinometer transmission, (Ef/Ei)act, which was measured similarly by placing the power meter head either before the photolysis cell (position no. 1)or following it (position no. 2) with the cell filled with actinometer solution. The laser beam in both cases underfilled the sensor surface. By Fresnel’s laws the transmittances of the entering and exiting windows a t normal incidence are identical, and the transmittance per window, T,, is The Journal of Physical Chemistry, Vol. 80,No. 20, 1976
where the bar over each term in braces indicates the function’s average value during photolysis. For constant light intensity, [TME] decreases linearly with time, and becomes
[TME] 8=p+[TMEl [TME] = ([TMEIi
+ [TME]f)/2
where the subscripts i and f denote the initial and final concentrations, respectively, and Vi,, is the volume of solution irradiated. Equation 18 is only approximate, but even for the extreme case of [TME]f = [TME]i/B = 0.05 M, the difference between eq 18 and the exact solution is only -0.2%. The E,, correction arises because even though the solution is always under the same 02 partial pressure, 02 does not redissolve as rapidly as it is consumed. Thus, the average value of the E,, term will be smaller than if it were calculated using [021sat. Thus, [ 0 2 ] and 4’obsd will decrease with time during a continuous irradiation. This error source, denoted the oxygen-debt error, varies with the rate and length of photolysis. Fortunately, because K,, is large, it is usually simple to keep the variation of E,, within 1-2% of the value calculated using [ 0 2 I s a t . E , , is given by
where [ 0 2 ] t is the time dependent 0 2 concentration and [ozl the average concentration during photolysis. The approximate form is typically accurate to better than 0.2%. For a photolysis broken up into several intervals with partial equilibration between intervals, ti,,. is the total irradiation time and [ O Z ] ~ becomes a discontinuous function. As will be shown, partial equilibration minimizes the oxygen-debt correction. [O,] is evaluated from the time dependence of the 0 2 volumes consumed during irradiation and the total 0 2 consumed. For simplicity data are taken at even time intervals. If the total irradiation is broken into J subirradiationsofequal period with partial equilibration between intervals, [O,] in eq 19 can be estimated from
225 1
Laser Intensity Measurements by Chemical Actinometry
where h V O z ( t l )is the total volume consumed from the beginning of the experiment until t , of the hth subirradiation. hV(t,)is monitored at N 1 readings taken at even times for each subirradiation, including a t the beginning ( t o = 0) and the terminus ( t =~tlrr/J),The amount of 0 2 reacted is assumed linear with photolysis time, an excellent assumption under our experimental conditions. For J = 1eq 20 reduces to the case of a single irradiation period. Procedure for Determining $'ob&. The experimental procedure was as follows: 150 ml of stock T M E solution (-0.12 M) and the solid weighed [Ru(bpy)s]C12.6H20were added to the cell. This solution was oxygenated by bubbling through -0.5-1 1 of solvent-saturated 02 (two methanol bubblers) while magnetically stirring; the cell was immediately tightly stoppered. After oxygenation of the actinometer solution, the bubblers were removed, and the gas buret filled and flushed several times with pure soluent free 0 2 ; the 0 2 connection was made at the cell side of the capillary. The buret was refilled with 0 2 with the leveling-bulb level set near the bottom of the buret, and stopcock G was closed. The capillary and stopper were then quickly inserted into the photolysis cell. Stopcock G was opened to equalize pressure, the fluid level within the buret and leveling bulb was equalized, and the stopcock closed. A buret reading was then taken. The opening, leveling, closing, and reading procedure was repeated at 5-min intervals until the gas volume remained constant (50.05 ml) for at least two readings. Unless room temperature underwent large fluctuations, no more than 30 min was required. Immediately before beginning irradiation, a bolometer reading was taken at position no. 1. The bolometer was then placed at position no. 2 to measure transmitted radiation, and the photolysis begun. Uptake was monitored a t 1- or 2-min intervals during the photolysis by leveling the buret. Immediately after photolysis, the bolometer reading was repeated at position no. 1.l a b s (eq 11)was calculated using the readings at the beginning and a t the end of the photolysis and the average value was used to calculate $'&d in eq 11;the agreement between the initial and final I a b s was usually ,
