2806
Anal. Chem. 1984, 56,2806-2811
Pulsed Infrared Laser Thermal Lens Spectrophotometric Determination of Trace-Level Gas-Phase Analytes: Quantitation of Parts per Billion Dichlorodifluoromethane George R. Long and Stephen E. Bialkowski* Department of Chemistry and Biochemistry, UMC 03, Utah State University, Logan, Utah 84322
Thermal lens spectrophotometry employlng a pulsed TEA-CO, laser excltatlon source Is applled to quantlflcatlon of trace amounts of dlchlorodlfluoromethane. The high radiation flux density or lntenslty of thls eTcltatlon source results In optlcal saturation of the Infrared transltlon. The resultlng slgnals show slgnlflcant devlation from the slmple theories used to describe the thermal lens process. The detectlon llmlt of dlchlorofluoromethane observed In these experiments Is 10 ppbv In argon at a total pressure of 13.3 kPa (100 torr), whlch Is extrapolated to an atmospheric pressure detection llmlt less than 1 ppbv. These detection llmlts correspond to 0.3 pg of difluorodlchloromethane in the lrradlated volume. The detectlon llmlt was found to be bound by slgnals due to gas hpurltles, wlndows, and most likely hot band transltlons of CO, in air. Suggestlons for further Improvements on the experimental setup are dlscussed.
Thermal lens spectrometry (TLS) is one of several calorimetric techniques used for high-sensitivity spectrophotometry ( I ) . Since the first reported recognition of the thermal lens effect in 1964, there has been much advancement in the theory (2-6) and practice (6-8) required for analytical application of this technique. For an analyte that is weakly fluorescent, electromagnetic energy absorbed and not lost by subsequent emission will result in heating of the medium. The TLS signal is derived from a change in the refractive index resulting from this heating. Because TLS is dependent on “dark” relaxation mechanisms, it serves as a complimentary technique to the very sensitive laser excited fluorescence technique, where the signal is derived from the fluorescence of the excited analyte. The TLS signal, on the other hand, is derived from the defocusing of an optical beam as it propagates through a medium with a radially symmetric refractive index gradient. This index gradient is a consequence of the temperature rise induced by thermalizing relaxation of the excited state. TLS is thus a second-order effect where the optical signal is not necessarily derived from the radiation exciting the transition but can be derived from a second light source which “probes”the thermal lens. This fact has lead t o the development of optical configurations that have evolved along two different lines ( I , 7). The single-laser optical configuration uses a continuous wave laser to both excite the analyte and probe the resulting thermal lens. The two-laser design uses one laser to excite the analyte and a second laser to probe the thermal lens. The sensitivity of the single-laser configuration is less than the two-laser one, but it has the advantage of being easier to align (7). There are advantages to the two laser configuration which make it a more convenient choice when dynamic or spectroscopic measurements are to be made ( I ) . The advantages to this technique are (1) the pump-laser wavelength can be varied so that excitation spectra can be obtained (9),(2) high-intensity pulsed lasers can be utilized for multiphoton and 0003-2700/84/0356-2806$01.50/0
multiquantum excitations (2), and (3) the same detector is used for all wavelengths of the pump beam, thus infrared excitations can be monitored with visible detectors ( I O ) . The advantages of the latter make this technique much more attractive for general application to chemical problems. The application of TLS to analytical problems has for the most part been limited to the detection of stable visible-ultraviolet absorbing analytes in solution. Gas-phase TLS has been used mostly for chemical dynamics studies ( 2 1 , I 2 ) , although recently Mori et al. (13) have applied TLS to the quantification of NOz in the gas phase and Carter et al. (8) have used infrared CW laser excitation. The former study showed that the use of a pulsed laser in gas-phase TLS can result in a significantly enhanced signal. The pulsed-laser thermal lens signal may be expressed as (15)
S = Ea(1
+ 2t/tJ2
(1)
where E is the enhancement factor, a is the e based absorbance, and t, is the characteristic time constant, w z / 4 K , where we is the excitation laser beam waist and K is the thermal diffusion coefficient. The enhancement factor for pulsed-laser excitation is given by
where E , is the pump-laser energy, X is the probe-laser wavelength, w, is the probe-beam radius at the focus, p is the molar density of the gas phase sample, C, is the molar heat capacity, and n is the refractive index. In contrast to the CW-laser enhancement factor (4),the pulsed-laser enhancement factor does not depend on the thermal conductivity of the sample, and it is inversely proportional to the probe laser waist squared. These two facts result in a calculated enhancement which is much greater for the pulsed laser with an equivalent average energy. Further, the enhancement for the pulsed-laser TLS is relatively independent of the choice of solvent or buffer gas. This minor dependence is due to the variation of dn/dT. This factor is lowest for He but does not vary by more than a factor of 2 for most other light gases. Another important difference between the CW-laser and pulsed-laser excited TLS is that of the rise time of the signal. The signal rise time for CW-laser TLS is on the order of a few t, (I), that is, on the order of milliseconds. In sharp contrast, the pulsed-laser TLS rise time is on the order of either the relaxation time or the acoustic transition time (3). These times are typically in the microsecond to nanosecond range. In this paper we examine the use of a pulsed TEA-C02laser as the excitation source for the two-laser TLS determination of dichlorodifluoromethane. Dichlorodifluoromethane has a strong absorption accessible to the CO, laser, and quantification of trace amounts in gas-phase samples is an important problem in analytical chemistry (14). Thus, this species serves as an ideal candidate for an exploratory investigation of gas-phase TLS utilizing pulsed infrared laser excitation 0 1984 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 He.Ne
TELESCOPE
MIRROR
LASER
CONCAVE
GERMANIUM
a’
&
LM741C
I
I
Figure 1. Optical schematic of the gas-phase, infrared trometer used in these experiments.
TLS
spec-
sources. The pulsed energy of the excitation source used in this study was sufficient to cause transition saturation. There are several effects that saturation has on the resulting TLS signal that make it a desirable phenomena for analytical purposes. We have found that the signal continuously increases with inert gas pressure, the pulse-to-pulse energy fluctuations do not manifest themselves linearly in the signal, and that the signal is greater than that obtained below the saturation energy. Argon is used as the buffer gas in this study. It’s temperature-dependent refractive index change is similar to that of air (13) and can be obtained in high purity and of singular composition.
EXPERIMENTAL SECTION A diagram of the two-laser thermal lens spectrometer used in these experiments is shown in Figure 1. The infrared laser used to excite the analyte was a TEA-C02laser constructed in this laboratory. The repetition rate of the pulsed laser was 3.75 Hz, and the firing command pulse was synchronized to the 60-Hz ac line in order to minimize effects due to line interference pickup. This laser had a hemispherical optical resonator with a plane diffraction grating and a concave Ge output coupler. An adjustable iris was placed near the grating in order to control the transverse mode characteristics of the output beam. The aperture of the iris was made as small as possible in order to force TEMWmode operation. The beam quality was inspected with the use of Optical Engineering phosphorescent beam probes and a graphite paddle. Every attempt was made to ensure a Gaussian beam profile during the course of the measurements. The laser pulse energy was about 100 mJ. The pulse was about 170 ns fwhm in duration with a typical 3-ps “tail”. Poly(ethy1ene)sheets were used to attenuate the laser energy. Pulse-to-pulse energy fluctuations were large. This fluctuation was typically &lo%. The beam radius at the output coupler was about 0.25 cm. The infrared laser beam was collimated about 4 m from the output coupler. A Keplerian telescope was used to reduce the beam diameter by a factor of 2.3 before mixing with the 632.8-nm HeNe laser beam at the Ge flat. Roughly half of the infrared energy was transmitted through the Ge flat. The reflected portion was utilized to monitor the energy of this laser. Some distortion of the infrared beam occurred at this Ge flat. In particular, linear interference fringes were observed. A Laser Precision Model RjP-735 energy monitor was used to detect the infrared laser energy. The infrared laser and the 5-mW Spectra Physics HeNe laser beam, mixed at the Ge flat, then propagated about 1 m to a wide aperture concave reflector having a focal length of 81 cm. Both beams were focused but at different distances from the focusing reflector. The stainless steel sample cell was placed at the focus of the infrared laser. This cell was 7 cm long and 4 cm i.d. and was fitted with Harshaw laser-grade NaCl windows. The cell was wrapped with heating cord so that bakeout under high vacuum Pa) could be performed. The COz laser spot size was determined by
2807
scanning a razor blade across the beam. It was found that this focus had spot size of 6 X low4m. The HeNe laser beam waist in the cell was 3.8 X lo-‘ m and was focused about 0.5 m past the m. Changes in the intensity of cell to a spot size of 2.2 X the HeNe laser beam passing through a 0.06 mm radius pinhole placed at distances from 0.1 to over 1 m from the cell were observed with an EG & G Model SGD-040-APIN photodiode biased to -9 V. The diode signal was amplified with an ac-coupled LM741C operational amplifier before being recorded with a Physical Data Model 522A 20-MHz, 8-bit transient digitizer. Signal rise times were limited by the differentiating amplifier used in these experiments. The band-pass of this amplifier, due to the product of the input rc characteristic and the high-frequency roll off of the LM741C, resulted in a rise time of about 50 ps. Data collection was handled by a DEC LSI 11/2 microprocessor interfaced to the transient digitizer. Both multichannel averaging and pseudo-gated integration incorporating the transient digitizer were used to obtain the measurements. In the multichannel averaging mode, the entire signal was summed into consecutive memory channels upon each pulse of the COzlaser. The resulting data were stored for later analysis. This subsequent analysis was performed by using a graphics cursor controlled integrative routine. The signal base line and maximum differences were the important parameters. Since signal averaging was performed, the precision of this data was related to the SIN of the averaged result The pseudo-gated integration routine utilized two user-defined gate regions: one for the base line, the other for the signal, which were selected with a graphics curser. Upon each COz laser pulse, the average base line, obtained by summing the data within the base line gate and corrected for the number of data, was subtracted from the average signal, obtained in an equivalent fashion. The precision of data obtained with this routine was displayed along with the average. Also recorded was the laser pulse energy. The output of the energy monitor was sampled with an ADAC A/D converter. When the pseudo-gated integrator routine was used, the mean and standard deviation of the COz laser pulse energy could also be displayed. The time difference between the base line and signal measurements varied with total gas pressure. This difference was typically 100 ps. In these experiments the infrared laser operated on the P32 line of the 10.6-pm transition at 933 cm-I (13). Because the transition was saturated at the higher intensities used in these experiments, great care had to be taken to ensure that the signal was that due to thermal lensing and not photothermal deflection. To accomplish this, the signal was observed as a function of pinhole position along the radial axis of the beam. A thermal lens signal yields a symmetric pattern when translated while that of a deflection signal will be asymmetric. Obtaining the TLS signal was relatively easy since all optical elements were mounted on translational stages. The argon used in this study was Matheson ultra high purity (99.999%) and the dichlorofluoromethane (freon 12) was PCR research grade (99%). Both were used without further purification.
RESULTS AND DISCUSSION In the simple theories describing TLS signals, the probelaser-beam parameters are changed by a thermal lens of focal length, fo. The focal length is calculated from the curvature of the refractive index gradient produced by the absorption of energy from the exciting laser beam. The inverse focal length of the thermal lens is (12) (3)
where f o is the minimum focal length, a is the exponential absorption coefficient, 1 is the cell length, and no is the refractive index. For pulsed-laser excitation, this focal length will change with time. The on-axis temperature change is in fact a function of the rates of mass and thermal diffusion and of vibrational energy relaxation, as well as the pulse energy and the specific heat of the gas (5). However, in the absence of multiphoton transitions and saturation effects, l/fois di-
2808
-2j
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 1.00,
81
I
d A
;;
Z -124
z ] W
-22
W >
0. 0
0. 5
1. 5
1. 0
1 2. 0
DISTANCE ( M I Flgure 2. Plot showing the experimental (x) and theoretical (-) signal as a function of detector distance from the sample cell for the beam parameters of these experiments. DETECTOR
rectly proportional to the excitation laser energy and the absorption coefficient of the analyte at a constant gas pressure. The signal observed through an infinitely small aperture pinhole located a t a distance d from the thermal lens is (2)
Io - I
s=-- IO
c
where zo is the Rayleigh range of the probe laser beam and z is the distance between the probe-laser-beam focus and the thermal lens. For small temperature changes, the focal length change is correspondingly small and terms second order in the focal length may be neglected. Optimization by maximizing the signal may be accomplished by optimizing the pump- and probe-beam parameters independently. The signal will be a maximum when the excitation laser beam has (1)a high energy, (2) is tuned to a maximum absorption of the analyte, and (3) has a minimum focus spot size. These conditions optimize the signal even if nonlinear effects are occurring. Optimum values of the probe-laser-beam parameters and detector placement can be found from eq 4. The optimum detector distance is found from the differential maximum of the signal with respect to this distance. Neglecting terms second order in focal length, the maximum signals occur a t the two distances -(22
dmax
=
+
202)
( 2 f 20)
(5)
These negative distances only can be realized if the probe laser beam focus is past the sample cell (Le., between the sample cell and the detector). The optimum cell-to-probe laser focus distance, z , can be found in the same fashion. Here setting the derivative of the signal, S, with respect to z equal to 0 yields the two roots , , ,z
= -(d
f 20)
(6)
Since both zo and d must be positive values, and d > zo, the optimum position of the probe-beam focus must be past the sample cell. These optimum beam parameters neglect the effects of the finite aperture size of the pinhole. These effects will be the theme of a future study, and preliminary calculations show that they are in fact substantial. A plot of the theoretical signal as a function of detector distance along with the experimentally determined values is illustrated in Figure 2. This figure does not include points near the focus of the HeNe laser since the criteria that the pinhole radius be less
0.00 -,7-----, 0 20 40
60
90
100
TOTAL GAS PRESSURE (kPa) Flgure 3. Plot of the calculated pressuredependent TLS signal. The
pressuredependentsignal (curve A) is first calculated by using typical vibrational relaxation rates and diffusion coefficients (see ref 5). The pressuredependent small signal absorption coefficient is measured experimentally and fit to an exponential function (curve B). The resulting signal is the product of the two pressure-dependent curves (curve C).
than the beam waist is not met in this case. The detector position used for all subsequent data was the experimentally obtained optimum value. Another useful approximation for determining the optimum beam geometrics can be found if it is assumed that the focal length of the thermal lens, f , is very long compared to the detection distance, d, and that the Rayleigh range is much less than d (2). In this case the signal is equal to -2d/f0, and the optimum probe laser beam geometry for a matched beam radius in the cell can be determined by substitution of the usual beam propagation equations (12). With these approximations the signal is a maximum when z = f3-1/2zo. The signal will change with total gas pressure because of the dynamic effects of energy relaxation and mass and thermal diffusions and because of the changing density (3, 5 , 10). Calculations of the pressure-dependent signal involve a numerical integration of the function
(7) where k is the rate constant for excited-state relaxation and t d is the characteristic time constant for mass diffusion equal to w 2 / 4 D , D being the mass diffusion coefficient and o,the excitation pulsed-laser-beam waist. The calculated thermal lens signal for these experiments is shown in Figure 3A. This signal reaches a maximum a t about 20 kPa (150 torr). This calculation was performed with the condition that the absorption coefficient, a, was not pressure dependent. The pressure-dependent cy is illustrated in Figure 3B and is discussed later. The signal is directly proportional to cy for small a, and the product of a with the theoretical signal should yield the total pressure-dependent signal. This product is illustrated in Figure 3C. Of importance here is the fact that one should observe a low pressure fall off in signal for gas-phase TLS. This low pressure fall off is due to the competitive dynamic effects of energy relaxation rate and diffusion rates. The diffusion rates increase with decreasing pressure while the relaxation rate increases with pressure. The results reported in this work are of the intensity changes that were recorded with the transient digitizer-ac coupled preamplifier configuration described in the Experimental Section. This signal is (8) Sexptl = VAZ = V ( I - Io)
ANALYTICAL CHEMISTRY, VOL. 56,NO. 14, DECEMBER 1984
2809
51 X
2
"
E
41
I
l
0
I 7 50
LASER ENERGY
100
150
(mJ)
Flgure 4. Plot of the measured TLS signal in millivolts as a function of the TEA-COP laser pulse energy in milllJoules. The dichlorodifluoromethane is 1 ppmv in 13.5 kPa of argon.
where V is the current-to-voltage amplification factor of the preamplifier. AZ is the absolute change in intensity detected by the photodiode past the pinhole. The signal defined in this fashion is related to the usual definition of AI/Io, by only a multiplicative constant. The fact that the signal was obtained over a very short (- 100 ps) time resulted in a more precise measurement. Signal variations due to short term intensity fluctuations of the probe laser were minimized both by fast signal collection and by signal averaging. The TLS signal was measured as a function of analyte partial pressure, totalgas pressure, and infrared laser intensity. A plot of the infrared laser energy-dependent TLS signal is shown in Figure 4. At the infrared laser pulse energies used to obtain this data, the TLS signal varies linearly with the laser energy, However, this plot does not have a zero intercept as would be expected from Beer's law behavior. The nonzero intercept with a small slope indicates that transition saturation is occurring. Therefore, the increase in laser pulse energy past the saturation energy does not significantly enhance the signal. The apparent saturation behavior can be used to an analytical advantage. When the TLS measurements are performed at an energy above the saturation energy, signal fluctuations due to the pulse-bpulse energy fluctuations of the TEA-C02lasers were minimized. Other implications of saturation are manifested in the pressure-dependent data since saturation is a pressure-dependent phenomenon (17). Increasing the inert gas pressure will increase the analyte absorbance due to pressure broadening. The pressure-dependent absorbance was measured by using the Laser Precision energy monitor. These data were fit to an exponential function by using the phase-plane method (19). The atmospheric pressure absorption coefficient calculated from these data for dichlorodifluoromethanein argon was 35.8 (atm cm)-'. This value corresponds well to the value of 35.7 (atm cm)-' obtained by Mayer et al. for dichlorodifluoromethane in air a t atmospheric pressures (14).The phase-plane fit of our data is illustrated along with the theoretical signal in Figure 3. Figure 5 illustrates the effect of inert gas pressure on the signal. The line drawn through the data is again that obtained from the exponential phase-plane method. The fact that the signal increases across the entire pressure range studied is most likely a manifestation of the saturated transition. It has been shown that increasing pressure increases the saturation intensity (17). This is the result of the greater rate of collisional deactivation at these higher pressures. If the laser intensity is sufficient to cause saturation at all pressures, then an increase in absorption will occur with increasing pressure throughout the pressure range. This will result in
TOTAL GAS PRESSURE (kPa) Flgure 5. Plot of the signal in millivolts vs. argon gas pressure at
constant dichlorodifluoromethanegas pressure. The dichlorodifluoromethane pressure was about 1 Pa, and the infrared iaser energy was about 120 mJ/pulse. 30
1I
25 4
5k-p7v7
00.0
2. 5
5. 0
FREON-12 CONCENTRATION
7. 5
10.0
(pprnv)
Feure 6. Working curve of the measured intensity change in millivolts vs. the dichlorodifluoromethaneconcentration at 13.3 kPa total pressure in argon. These data represent several experiments and are linear over a concentration range from 10 ppbv to about 7 ppmv.
an analytical advantage at high pressures, and, therefore, trace analysis of atmospheric pressure samples will be possible. The working curve for dichlorodifluoromethane in 13.3 kPa of argon is shown in Figure 6. Signals at the lowest concentrations, 1.3 X lo4 and 1.3 X lo-' kPa, were signal-averaged 1000 times. All other signals were obtained by using 250 averages. A large background signal was observed during these experiments. To account for this, TLS signals were obtained with 13.3 kPa of pure argon in the cell. This signal was then subtracted from the signals obtained from the samples containing dichlorodifluoromethane, The background signal was the limiting factor in detecting small sample concentrations. The limit of detection of dichlorodifluoromethane in 13.3 kPa of argon is 10 ppbv. This detection limit may be extrapolated to 1 ppbv at atmospheric pressure as per Figure 5. TLS detection limits may also be expressed as the amount of material in the volume of the laser beam, in this case 0.3 pg. The latter detection limit compares favorably to other methods used for halocarbon trace analysis, such as the electron capture detector which has a detection limit of 1.4 pg (16). These results are compared to other techniques in Table I. Although these measurements were obtained in argon, the results are directly comparable to those obtained in air. The theoretical enhancement for pulsed-laser TLS depends only on the refractive index change with temperature. This change is only 2% greater for air than for argon (13). Thus, only a small (2%)
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table I. Comparison of Minimum Detectable Dichlorodifluoromethane by Several Techniques" method infrared absorptionb electron capture/GCc LIBSd this worke
mass
concn
NA
6 PPb
1.4 Pg 5.0 ng 0.3 Pg
3 PPt 16 PPm
1 PPb
"Detection limits at standard pressure, 100 kPA. bFor 1 km path length C 0 2 laser absorption in air, ref 13. 3 parts per trillion in 100 mL of air, ref 15. dLaser-inducedbreakdown spectrometry. Mass detections limits in helium, concentrations in air: Cremers, D. A,; Radziemski, L. J. Anal. Chem. 1984, 55, 1252. e Concentration limit extrapolated from 13.3 kPa to standard conditions in argon.
decrease in detection limit should be observed for similar measurements in air. This study shows that TLS using a high-powered IR laser is a promising technique for trace analysis. The apparatus discussed here, while giving detection limits comparable to other techniques for halocarbon detection, is by no means optimum. Elimination of the background signal would result in at leaBt an order of magnitude decrease in the detection limit. The background signal may be due to ambient atmospheric absorptions, absorptions by the NaCl windows, or contamination of the sample from material absorbed on the walls of the sample cell. Using a short focal length lens to focus the COz laser into the cell would reduce the intensity at the cell windows and hence reduce the signal from any window absorptions. Mixing the pump and probe beams as close to the cell as possible will also reduce background signals due to atmospheric absorptions. If the background signals are effectively eliminated the main limitations on the TLS signal will then be the pulse-to-pulse energy variations of the infrared laser and the instrumental noise that occurs in the amplification and digitization of the small signals. The treatment of saturation of infrared transitions is complicated owing to the large number of rovibrational energy levels and the occurrence of hot band transitions (17). However, many of the features of a saturated transition may be qualitatively described by a simple model. For a two-level system, the saturation intensity is defined as (20) hv 2u7
I, = where hv is the transition energy, u is the absorption cross section, and 7 is the lifetime of the upper state. Radiative lifetimes of infrared transitions are on the order of 10 ms. On the other hand, the collisional vibrational relaxation time constant is about 1.1ps at 13.3 kPa (I1,12).Thus, the saturation intensity will be directly dependent on pressure in the range of pressures utilized in this study (21). The pressuredependent saturation intensity will modify two terms of importance to the calculation of the TLS signal; the curvature of the refractive index and the amount of energy deposited in the sample cell. Both parameters affect the focal length of the thermal lens. The pressure-dependent saturation intensity calculated for dichlorodifluoromethane based on the atmospheric pressure absorption coefficient (14) is 450 W kPa/cm2. The intensities utilized in these experiments were between a few hundred kW/cm2 and several MW/cm2. It is probable that saturation was occurring to a great extent. One effect of saturation will be to decrease the curvature of the refrative index gradient at times short compared to thermal diffusion. The population for short times after the excitation laser pulse distribution for a Gaussian beam profile will be described by the equation
r
. I
(10)
where F is the fraction of species in the excited state. E , is the pulse energy, t, the pulse length, and we the beam waist radius of the excitation source. The intensity of the excitation source may be defined as
Z = 2E,/tPu,27
(11)
and the on-axis (r = 0) curvature of the excited fraction is proportional to the inverse focal length of the thermal lens. Taking the second derivative of eq 10, evaluating at r = 0, and using eq 3 and 11, one may obtain
where C is the concentration, 1 is the cell length, and fo is the focal length at t = 0. The results shown in eq 12 qualitatively demonstrate the effect of intensity on the thermal lens signal, which is proportional to l/fo. At zero pulse energy the focal length is infinity; that is, there is no lens. As the intensity increases, but remains less than I,,the focal length will decrease. In fact, for low intensities
z > I,
(e))"
_1 -- 2hvCl f o nopCpo,P dT
Thus, the signal will decrease with increasing intensity when the excitation source intensity is much greater than Is. This decrease in the curvature is not due to a decrease in the number of excited species. Quite the contrary, as the intensity is increased, more species are excited to the upper level but not necessarily on axis. More species are excited to a saturated fraction of 0.5 off axis, resulting in a flatter index profile and thus a reduced curvature. This flattening of the profile, while increasing the total number of excited species with increasing intensity will result in a index gradient exceeding that obtainable with a diffraction limited laser spot. This fact can be utilized to increase thermal deflection signals (18). In fact, the alignment of the two beams in these experiments was difficult owing to the fact that the thermal lens signal was less than the collinear thermal deflection signal. The theory predicts the opposite behavior; that is, thermal lens signals should be greater than those of thermal deflection (17). However, the high index gradient, as well as the low index curvature, will dissipate with time because of thermal diffusion. The Green function for thermal diffusion is Gaussian ( Z ) , and, in time, the curvature of the saturation flattened refractive index will increase, while the index gradient will decrease. At still longer times, both the curvature and the gradient will dissipate as the heat is distributed throughout the cell. The increase in the curvature of the refractive index gradient due to thermal diffusion may account for the high sensitivities realized in these experiments and the increasing signal with intensities well beyond that of saturation. Alternatively, the observed increase of signal with intensity may be due to multiphoton excitation of the analyte.
Anal. Chem. 1984, 56, 2811-2815
The intensity dependence of multiphoton absorption is P , n being the number of photons absorbed (2). These effects would not be expected to be important until the higher intensities since the cross sections for multiphoton transition are much lower than those of the single photon. The increase in signal with increasing pressures may be explained by the saturation phenomenon. With the high laser intensities that were used in this study, it is very likely that eq 14b describes the pressure-dependent focal length. The two pressure-dependent terms, dn/dT and p , are both directly proportional to pressure (IO). The pressure dependence of the TLS signal should then be due to the pressure dependence of I, only. The zero-time saturation signal does not give the maximum signal. But, the maximum signal trend should follow the zero-time signal. Since I , is proportional to the pressure, the pressure-dependent signal for intensities above I , should also increase with pressure.
CONCLUSION In conclusion, this study shows the viability of the use of high-energy COz lasers in TLS. In the future infrared TLS may be used to gain both structural and quantitative informations about analyte species and data. Infrared absorptions previously unaccessible to conventional IR spectrometers may be studied since TLS can measure absorbances < lo-’ (atm cm)-l. These things, along with the relative simplicity of TLS instrumentation, make this a very useful tool for studying future analytical problems. We are currently developing a more precise mathematical description of the saturation thermal lens and thermal deflection signals. Thermal deflection experiments under saturation conditions are also being performed. Preliminary results indicate that the detection limits for gas-phase infrared absorbers can be decreased by at least an order of magnitude
2811
over those of the saturation thermal lens experiments described here. Registry No. Dichlorodifluoromethane, 75-71-8.
LITERATURE CITED (1) Fang, H. L.; Swofford, R. L. In “Ultrasensitive Laser Spectroscopy”; Kiiger, D. S., Ed.; Academic Press: New York, 1983; Chapter 3. (2) Twarowski, A. J.; Kiiger, D. S. Chem. Phys. 1977, 20,253-258. (3) Barker, J. R.; Rotherm, T. Chem. Phys. 1982, 68, 331-339. (4) Carter, C. A.; Harris, J. M. Appl. Opt. 1984, 23,476-481. (5) Bialkowski, S. E. Chem. Phys. Lett. 1984, 104, 448-454. (6) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52,2338-2342. (7) Carter, C. A.; Brady, J. M.; Harris, J. M. Appl. Spectrosc. 1982, 36, 309-314. (8) Carter, C. A.; Harris, J. M. Anal. Chem. 1983, 55, 1256-1261. (9) Long, M. E.; Swofford, R. L.; Albrecht, A. C. Science (Washington, D . C . ) 1976, 191, 183-185. (10) Bailey, R. T.; Cruikshank, F. R . ; Pugh, D.; Johnstone, W. J . Chem. SOC. Faraday Trans. 2,1980, 633-647. (11) Xing-Xiao, M.; Zhu-De, X. Chem. Phys. Lett. 1983, 96, 563-565. (12) Siebert, D. R.; Grabiner, F. R.; Fiynn, G. W. J . Chem. Phys. 1974, 60, 1564-1574. (13) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1075-1079. (14) Mayer, A.; Comera, J.; Charpentier, H.; Jausaud, C. Appl. Opt. 1978, 17, 391-393. (15) Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 9 , 1663- 1669. (16) Madjar, C. V.; Parey, F.; Excoffier, J. L.; Bekassy, S. J . Chromatogr. 1981, 203,247-261. (17) Wood, R. R.; Gordon, P. L.; Schwarz, S. E. IEEEJ. Quantum. Electron. 1969, 10, 502-513. (18) Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1981, 20,1333-1343. (19) Bacon, J. R.; Demas, J. N. Anal. Chem. 1983, 56,653-656. (20) Sveito, 0. “PrinciDles of Lasers”, 2nd ed.; Plenum Press: New York, 1982; pp 58-68.. (21) Fiygare, W. H. Acc. Chem. Res. 1968, 1 , 121-127,
RECEIVED for review June 22,1984. Accepted August 9,1984. This work was supported by a grant from Utah State University for faculty development. Much of the equipment was purchased through a grant from Research Corp.
Sensitized Room Temperature Biacetyl Phosphorescence via Molecular Organization Frank J. DeLuccia and L. J. Cline Love* Seton Hall University, Department of Chemistry, South Orange, New Jersey 07079
Room temperature sensitized blacetyl phosphorescence enhanced via molecular organlratlon Is observed for many aromatlc compounds. Micelles composed of sodium dodecyl sulfate (SDS) and hosts such as p-cyclodextrln (p-CD) are used to enhance the energy transfer reactlon by organlzing the reactants In close proxlmlty to one another. The trlplettrlpiet energy transfer of several polynuclear aromatics, nltragen heterocyclics, and heavy-atom-substltuted species to blacetyl was evaluated. Both SDS and p-CD were found to provide a superior medlum for production of sensltired blacetyl phosphorescence than found with homogeneous soiutlon. Typically, limlts of detectlon of 1 X lo-* to lo-’ M In SDS and 1 X IO-’ M In P-CD were obtained. The sensltivltles of the methods are dependent on the solublllty of the donor in SDS, the ablllty of the donor to fit Into the 0-cyciodextrln cavlty, and the background slgnal produced by the blacetyl blank solution.
Recent developments in solution chemistry have made it possible to observe triplet state emission from a large number
of polynuclear aromatic (PNAs) compounds in fluid solution at room temperature. Cline Love and co-workers have demonstrated that excited phosphorescent compounds associated with micellar assemblies (1,2) or included into cyclodextrin cavities (3, 4) will deactivate via triplet state emission. In addition, phosphorescence was shown to occur in colloidal suspensions where the insoluble solute interacts with its crystalline neighbor (5). Each of these techniques requires a high degree of molecular organization to stabilize the triplet state and induce room temperature phosphorescence (RTP). An alternate method for studying the triplet state processes of molecules is by energy transfer interactions. Sensitized phosphorescence involves triplet-triplet energy transfer according to Donkerbroek’s eq 1 (6)
where D is the donor molecule, A the acceptor molecule, and So and TI are the ground state and excited triplet state, respectively. Almgren and co-workers studied the energy transfer from triplet state aromatic hydrocarbons to Tb3+and Eu3+ions in
0003-2700/84/0356-2811$01.50/00 1984 American Chemical Society
