J. Phys. Chem. 1988, 92, 5954-5958
5954
Figure 6. The structure of cresyl violet.
with the less pronounced state-dependent reorientation dynamics observed experimentally. It is interesting to note that cresyl violet, a molecule structurally similar to oxazine 118, does not exhibit state-dependent reorientation properties in a wide variety of protic s o l ~ e n t s . ~ Cresyl ~-~2~ violet differs from oxazine 118 by containing an extra fused ring (see Figure 6). The absorption spectrum of cresyl violet has a maximum very close to that of oxazine 118. This added ring contributes little to the position of the absorption spectrum but does make it broader and more featureless. These effects have been attributed to steric interference of cresyl violet's end amino group by the additional ring structure.24 It is likely that steric blocking of the ring-bound nitrogen by this same ring accounts for the lack of state dependence reported in reorientation measurements.
Conclusions The rotational diffusion behavior of oxazine 118 in the series of butanols is observed to be highly state dependent. This state dependence is not resolved in the solvent 2-butanone, suggesting that the alcohol proton plays a role. The state-dependent reorientation of resorufin in the butanols is much more subtle than
for oxazine 118. The absorption spectrum of resorufin is observed to be solvent dependent, indicating a strong interaction of the solvent alcohol protons with its carbonyl end groups. No analogous solvent dependence of the absorption spectrum is seen for oxazine 118. Semiempirical MNDO calculations of the ground state and the first excited singlet state for both molecules show a significant increase in a-electron density at their ring-bound nitrogen. For oxazine 118 the ring-bound nitrogen becomes a stronger Lewis base on excitation. For resorufin this also occurs, but the carbonyl moieties are relatively strong Lewis bases in both states. Thus, on excitation, the increase in electron density at the ring-bound nitrogen is of less significance for resorufin than it is for oxazine 118. In conclusion, it is not possible to examine the role that ionic charge plays in orientational relaxation measurements in as straightforward a way as was intended originally. This is due primarily to the fundamentally different chemistry of the (C=O) and the (C-NH2) moieties. This work does report, however, unambiguous state-dependent rotational diffusion times for both resorufin and oxazine 118 in certain solvents. It also demonstrates that changes in electron density at a specific atom within the probe molecule can be responsible for significant changes in its local environment. Acknowledgment. We are indebted to A. S. Gozdz for his assistance with the emission measurements as well as with the MNDO calculations. The efforts of J. P. Heritage and P. Grabbe are greatly appreciated for their assistance with the MNDO calculations and for their generous donation of computer time. We are also grateful to R. Steppel of the Exciton Chemical Company for providing the sample of oxazine 1 18 chloride. Registry No. Resorufin sodium salt, 34994-50-8;oxazine 118 chloride, 53669-98-0.
Characterization of a Pulsed Supersonic Beam of Ammonia Monomer and Clusters Using the Hexapole Electric Field Kazuhiko Ohashi, Toshio Kasai, and Keiji Kuwata* Department of Chemistry, Faculty of Science, Osaka University, Toyonaka 560, Japan (Received: December 29, 1987)
An intense pulsed supersonic beam of ammonia monomer is focused and state selected by a hexapole electric field. The intensity of the focused beam is estimated to be 10'' molecules sr-' s-'. The state-selected monomer beam is characterized by comparison of its intensity with the numerical calculation of trajectories according to the Stark effect for two nearby levels, and the rotational state distribution is obtained. Ammonia clusters with n members ( n = 2-10) are found to be unfocused by the hexapole field. Thus, neither a symmetric-top structure nor a very large electric dipole moment is expected for ammonia dimer and larger clusters. The analysis on the nozzle-stagnation pressure dependence of the focusing behavior shows a method to estimate the fraction of clusters in the beam by the hexapole technique. N
1. Introduction So far, the rotational state distribution of ammonia molecules in a supersonic beam or a free jet has been determined by spectroscopic methods such as IR absorption,14 coherent anti-Stokes Raman scattering (CARS),Smultiphoton ionization (MPI)? and
bolometric detection.' Due to small number density in molecular beam experiments, those methods often have a common difficulty in practical application. The electrostatic state selection may be an alternative technique to determine the rotational distribution for such a polar symmetric-top m ~ l e c u l e . * ~ ~
(1) Snavely, D. L.; Colson, S. D.; Wiberg, K. B. J . Chem. Phys. 1981, 74, 6975. (2) Baidacchini, G.; Marchetti, S.;Montelatici, V. Chem. Phys. Left. 1982, 91, 423. (3) Mizugai, Y . ;Kuze, H.; Jones, H.; Takami, M. Appl. Phys. B 1983, B32, 43. (4) Veeken, K.; Reuss, J. Appl. Phys. E 1984, 834, 149.
( 5 ) (a) Huisken, F.; Pertsch, T. Chem. Phys. Lett. 1986, 123, 99. (b) Barth, H. D.; Huisken, F. J . Chem. Phys. 1987, 87, 2549. (6) (a) Kay, B. D.; Grimley, A. J. Chem. Phys. Lett. 1986,127, 303. (b) Kay, B. D.; Raymond, T. D. Chem. Phys. Left. 1986, 127, 309. (7) Dam, N.; Liedenbaum, C.; Stoke, S.; Reuss, J. Chem. Phys. Lett. 1987, 136, 73. ( 8 ) Chakravorty, K. K.; Parker, D. H.; Bernstein, R. B. Chem. Phys. 1982, 68. I .
0022-3654/88/2092-5954$01 SO10
0 1988 American Chemical Society
Supersonic Beam of Ammonia Monomer and Clusters Owing to the molecular inversion which splits the degenerated K-levels, the inversion doublets of the ammonia molecule have to be described by t h e Stark effect for two nearby levels instead of a first-order effect.I0 Because this Stark effect can be approximated in the form of a second-order effect in weak electric fields," beams of ammonia molecules have been focused extensively by q u a d r ~ p o l e ~ or ~ , o~c~t 0- 'p~o l e ~ ~fields 3 ~ ~ as maser focusers. Butkovskaya et al. carried out experiments on the focusing of ammonia by the hexapole field." A recent study of Gandhi et al. has also shown the focusing of an ammonia beam by the hexapole.18a The focusing behavior has been discussed in terms of the velocity dependence of the threshold voltage.lgb Klaassen et al. have done the trajectory calculation of the ammonia molecules in the state selector using only one mean value for the velocity of the beam.I6 Exact trajectories are required to evaluate the rotational state distribution after state selection. It is particularly necessary to obtain such distributions to carry out the study of oriented beam reactions. In the present work, we focus the pulsed supersonic beam of ammonia and then characterize the state-selected ammonia beam by comparison of the focusing curves with the Monte Carlo simlat ti on.^ Trajectories are numerically calculated according to the Stark effect for the inversion doublet levels. The analysis on the nozzle-stagnation pressure dependence of the focusing behavior shows a method to estimate the fraction of clusters in the beam by the hexapole technique. Possible structures of ammonia dimer and clusters are also discussed.
2. Experimental Section
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5955
'
0.01
' ' 1 ' , ' ,
, ' I " ' ,
' " I , , ,
1
'
1
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The experimental apparatus to focus the N H 3 beam is similar to that of ref 19 with a change in the length of the hexapole rod to 60 cm. Neat NH, (purity 99.999%) was used after removing a trace of water by metallic sodium at dry ice-ethanol temperature. N o run with seeding gas was done in the present work. The supersonic N H 3 beam was produced by the pulsed valve of 1-mm diameter with stagnation pressures between Po = 10 and 600 Torr (1 Torr = 133 Pa). The beam was skimmed by the skimmer of 1.01-mm diameter. The most intense beam was obtained at a nozzle-skimmer distance of 15 mm at Po 200 Torr. The beam intensity was measured by the quadrupole mass spectrometer of electron-impact type (ULVAC MSQ-150A). In order to characterize the direct N H 3 beam, no axial beam stop was used. The velocity distribution of the direct beam was determined by time-of-flight (TOF) analysis. The mechanical chopper of ~ O - F Sgate was employed, and the flight length was 75 cm. The TOF profiles were averaged to improve S / N and compared with the calculated curves by using a stream velocity us and a translational temperature T, as adjustable parameters.20
-
3. Monte Carlo Trajectory Calculation The N H 3 molecule does not have the usual first-order Stark effect because of the splitting for the degenerated K-levels due (9) Kasai, T.; Ohashi, K.; Ohoyama, H.; Kuwata, K. Chem. Phys. Lett. 1986, 127, 581.
(10) Townes, C. H.; Schawlow, A. L. Microwaoe Spectroscopy; McGraw-Hill: New York, 1955; Chapter 10. (1 1) Mizushima, M. Phys. Reo. 1948, 74, 705. (12) Gordon, J. P.; Zeiger, H. J.; Townes, C. H. Phys. Reu. 1955.99, 1264. (13) Wang, J. H. S.; Oates, D. E.; Ben-Reuven, A,; Kukolich, S . G . J . Chem. Phys. 1973, SY, 5268. (14) Odutola, J. A.; Dyke, T. R.; Howard, B. J.; Muenter, J. S. J . Chem. Phys. 1979, 70, 4884. (15) Shimoda, K. J . Phys. SOC.Jpn. 1958, 13,939. (16) Klaassen, D. B. M.; Reijnders, J. M. H.; ter Meulen, J. J.; Dymanus, A. J . Chem. Phys. 1982, 76, 3019. (17) Butkovskaya, N . I.; Larichev, M. N.; Leipunskii, I. 0.;Morozov, I . I.; Tal rose, V. L. Chem. Phys. 1976, 12, 267. (18) (a) Gandhi, S. R.; Xu, Q.-X.; Curtiss, T. J.; Bernstein, R. B. J . Phys. Chem. 1987, 91, 5437. (b) Gandhi, S. R.; Bernstein, R. B. J . Chem. Phys. 1987,87,6457. The additional ref 18b was available after the first submission of this manuscript, which is important and relevant to the present work. (19) Ohoyama, H.; Kasai, T.; Ohashi, K.; Kuwata, K. Chem. Phys. Lett. 1987, 136, 236. (20) Wilcomb, B. E.; Haberman, J. A.; Bickes, Jr., R. W.; Mayer, T. M.; Bernstein, R. B. J . Chem. Phys. 1976, 64, 3501.
' 0 " I ' " '
(21) Jauch, J. M. Phys. Reu. 1947, 72, 715.
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5956
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
Ohashi et al.
80
i:L
.
0.0
0
200
400
600
E 3 *
-
0
0
200
400 P, / Torr
600
I
Figure 2. Dependence of the translational temperature T, of NH3 beam
on the stagnation pressure Po. The open circles show the experimental values determined by the TOF analysis. The solid curve connecting the data points is only for clarity. Calculated terminal translational temperature TT is indicated by the broken curve. according to eq 2. In the present study, we carry out the Monte Carlo simulationg including numerical calculation of trajectories in order to reproduce the focusing curve of NH, and to obtain its rotational state distribution as a function of Ip(, p K M / ( P J). Note that it is not possible to select the orientation of NH, (Le., the sign of p ) because the dipole matrix elements in eq 2 are quadratic and that it is only possible to make the N H 3 molecule aligned.
- 0
0
5
IO Vo/
kV
u 5 IO
oo
V,/W
Figure 3. (left) Dependences of the focused NH3 beam intensity on V, under cluster-freeconditions: Po = 10 Torr (A) and 100 Torr (B). The vertical scale is normalized to the beam intensity at V, = 0 kV on each Po;F(Vo,Po)= l(Vo)/Z(0).The solid lines stand for the best fits with T, = 78 K (A) and 42 K (B), respectively. The broken lines were calculated with T, = 88 K (A) and 52 K (B). (right) Podependences of the focusing curve under the cluster-forming conditions: Po = 200 ( C ) and 500 Torr
+
(D). The experimental points are normalized to the apparent beam intensity at Vo= 0 kV on each Po. The cluster-freefocusing curve at 100 Torr is referred with the dotted-dash curve (B). The inset shows the Po dependence of the parameter X obtained by fitting eq 5 to the experimental data. The resulting G(V,,Po)'s are indicated by the solid lines.
4. Results and Discussion
analysis are shown with open circles, and the expected terminal translational temperature TT is plotted with the broken line. TT was calculated from the following equationsz4
In the ionization of (NH,),, clusters, protonated-type ions are preferentially produced as given in eq 3;22meanwhile, the frag(NH,),
-
(NH3)mH++ ( n - m - l)NH3 + N H 2 + e-
(3)
mentation to the NH3+ ion concurrently takes place as a minor process, which is discussed in section 4.2. If we assume that a parent cluster detaches only a N H 2 fragment and not a NH3 fragment during ionization processes, Le., m = n - 1 in eq 3, the intensity of the protonated ion (NH,),]H+ reflects the intensity of its corresponding neutral cluster ("3)". Figure 1 shows the dependence of the peak intensities for the protonated ions (NH3),IH+ ( n = 2-10) as well as the dimer The parent peak (NH3)2+upon the stagnation pressure of "3. cluster with 10 members, which gives the (NH3)9H+peak, was the largest in size in the present mass-tuning range. The peak intensities for the clusters were growing in the range of stagnation pressure from 100 to 200 Torr. At 100 Torr, very weak peaks for the dimer and the trimer, NH4+ and (NH3)2H+,were able to be identified, so that the onset of the cluster formation seemed to be in the range from Pod = 10 to 20 Torr cm. A similar onset in the range from 17 to 22 Torr cm for the larger clusters (n > 4) was reported in the MPI study.22 This result is also consistent with the increase of the translational temperature, T,, due to the heat of cluster formation, which is discussed later in Figure 2. As shown by the solid lines in Figure 1, the Podependence of the peak intensity gives a slope with nth order and is fitted by the Po" scaling lawz3on expansions between 150 and 250 Torr. Hence, one may conclude that the (NHJW1H+ions mainly originate from the neutral (NH,), clusters with these stagnation conditions as we assume above. The natural abundance of nitrogen isotope 15N is 0.365%, so that care must be taken when we detect the dimer by measuring NH4+ intensity because the ISNH3monomer ion overlaps on the same mass number. The contribution of "NH3+ was estimated to be -7% of the observed intensity at m / e 18. Thus, we monitored both the NH4+ and the (NH3)2+ peaks to characterize the dimer. Figure 2 exhibits the Po dependence of the translational temperature T,. The experimental values of T, obtained by the TOF (22) Shinohara, H . J . Chem. Phys. 1983, 79, 1732. (23) Buck, U.; Meyer, H. Surf. Sei. 1985, 156, 275.
where To is the stagnation temperature, MT the terminal Mach number, and K,, the Knudsen number. The specific heat ratio for NH, was taken as y = 1.31,25the collision efficiency t as 0.25: and the effective molecular collision diameter as 0.43 nm. While the calculated TT shows a monotonous descent with rise of Poto reach 9 K at 600 Torr, the observed T, shows an S-shape variation, Le., initially decreases with increase of Po up to 150 Torr and then turns to increase to a maximum temperature of 70 K at around 350 Torr. This increase of T , may be due to the heat of cluster f ~ r r n a t i o n . ' ~At ~ ~ Po ~ - ~higher than 350 Torr, the translational temperature T, decreases again and merges with the calculated TTcurve. This behavior is explained by the fact that the cooling due to the expansion at the pulsed valve overcomes the heating by the cluster formation over 400 Torr. Consequently, at Po higher than 150 Torr the supersonic beam of NH, contains a considerable amount of clusters, while the beam practically consists of the monomer below this pressure. A strong focusing effect was observed for the NH, monomer by the hexapole field. On the other hand, the N H 3 clusters with n = 2-1 0 were unfocused within the experimental error of I ( 15 kV)/Z(O) = 1 f 0.05 at Po = 200 Torr. The dependence of the intensities of the focused beam upon the voltage applied to the hexapole rod is given in Figure 3. 4.1. The Focusing Curve for the Monomer Beam. Figure 3 (left) shows the focusing curves of the NH, beam at two different stagnation pressures of 10 and 100 Torr. The beam intensity was measured by the NH3+ peak height. The vertical scale for the focused beam intensity is normalized to the direct beam intensity at each Po. The mass spectra observed at these low pressures showed no cluster peaks. (24) Anderson, J. B. In Molecular Beams and Low Densify Gas Dynamics; Wegener, P. D., Ed.; Dekker: New York, 1974; p 1. (25) American Institute of Physics Handbook, 3rd ed.; Gray, D. E., Ed.; McGraw-Hill: New York, 1972; p 3.71.
Supersonic Beam of Ammonia Monomer and Clusters The trajectory calculations were carried out in order to simulate the observed focusing curves. The number of trial trajectories was 2 X los for each V,. Based on the results of the TOF analysis, the translational temperatures of T, = 78 and 42 K were used at Po = 10 and 100 Torr, respectively. The Boltzmann distribution was assumed for the molecular rotation of N H 3 in the supersonic The calculated focusing curves were sensitive to the rotational temperature of NH,. Judging from the threshold voltage and the slope of the focusing curves, the best fit was obtained when we took the rotational temperature equal to the translational temperature, namely, T, = T, = 78 K at Po = 10 Torr and 42 K at 100 Torr as indicated by the solid lines in Figure 3 (left). Veeken and Reuss reported that the rotational temperature of N H 3 was slightly higher than the translational temperature under free jet conditions, but they also stated that the rotational temperature dropped along the jet streams4 For reference the broken lines exhibit the focusing curves calculated with slightly higher TI than the corresponding T, by 10 K. With these higher T,, poor agreement with the experiments resulted. Kay and Grimley reported different rotational temperatures for the para and the ortho K-levels of N H 3 in the supersonic beam.6a Unequal terminal rotational temperatures for the two series of quantum numbers, J and K , were also measured by Douketis et al. in the free jet of CH3F-He mixtures.26 We could assume TI in this manner in the trajectory calculations, but it seems beyond the purpose of the present simulation to derive such detailed information. For simplicity, we used just one TI to express the rotational state distribution of the NH3 beam. The different trends of the focusing curves A and B were found to be entirely explainable only by the differences in TI,u,, and T, under cluster-free conditions. One can therefore conclude that the numerical trajectory calculation according to eq 2 is useful in the determination of the rotational temperature and thus the rotational state distribution of the focused N H 3 beam. 4.2. The Focusing Curvefor the Monomer plus Clusters Beam. As the stagnation pressure of N H 3 increases, the rate of enhancement in the focused beam drops gradually from B to D, as shown in Figure 3 (right). The following reasons could be considered when we increase the stagnation pressure: (a) increase in the stream velocity of the beam, (b) rotational cooling of the beam, (c) cluster formation, and (d) collisional state scrambling in the beam in the hexapole field. The effects of (a) and (b) for the cluster-free beams are briefly described in the previous section. It is necessary to discuss these effects in more detail in the Po region of the cluster formation. In general, as the pressure increases, the speed of the beam increases and the focusing becomes unfavorable. To the contrary, the observed v, decreases with increase of Po probably due to the cluster formation, which should be favorable for the hexapole focusing. Thus, (a) is not responsible in this case. Secondly, the rise of Pogenerally brings more effective cooling of rotation which accordingly changes the distribution of p; Le., the states around p = 0 are preferentially populated, which is unfavorable for the focusing. In the present case the rotational temperature should rise according to the rise of the translational temperature when the stagnation pressure is increased, as shown in Figure 2. Nevertheless, we observed the decrease of the focused beam intensity from B to D. Again, (b) is not the cause for the falloff in the focusing curves. We calculated the focusing curves at various TI and confirmed the above conclusion. With the increase of Po,the clusters begin to give additional contribution to the apparent NH3+intensity through fragmentation of the parent clusters. The test experiment using two different electron-impact energies of the ionizer ensured that the NH3+ions were not formed entirely from the monomer, giving uneven values of I( 15 kV)/Z(O) = 4.95 f 0.02 at 20 eV and 4.54 f 0.02 at 70 eV. Otherwise, those values should be identical regardless of the impact energy. Apparent I ( Vo)/I(0)values are thus underestimated because of the overestimation in the experimental Z(0) when (26) Douketis, C.; Gough, T. E.; Scoles, G.; Wang, H. J . Chem. Phys. 1984, 88, 4484.
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5957
0.0
1.0
IPI Figure 4. Calculated rotational state distribution of the NH3 monomer at Po = 100 Torr. IN denotes the distribution function before entering the hexapole field, and OUT is the one after the state selection at V, = 10 kV. A set of three quantum numbers in parentheses is given in the order of (J,14,1Ml).
Po is increased. Meanwhile, Z(0) should be the exact intensity of the N H 3 monomer in comparison with the calculated focusing curves. One can argue more quantitatively by the determination of the fractional contribution of the NH3clusters in the beam. We define F(Vo,Po)for the focusing curve under cluster-free conditions, which is obtained by the trajectory calculation, and G(Vo,Po) for the observed focusing curve in the presence of clusters. Unless the clusters are focusable, the following equation holds G(VoJ'0) = F(Vo,Po)(l
- x)
+X
(5)
Here a new parameter X is introduced to express the fractional contribution of the clusters to the NH3+ peak; hence, the quantity 1 - X gives the fractional contribution of the monomer in the beam. X can be determined by the least-squares method. For the beam with Po = 200, 350, and 500 Torr, X is derived to be 0.18,0.53, and 0.65, respectively. These values of X seem to be large regarding the one-step process for the NH3+ formation from the clusters. Two-step formation is very likely if we take into account the geometry of our mass spectrometer, in which a significant amount of neutral N H 3 molecules is produced from the parent clusters by the wall collision in the ionizer and the successive ionization of N H 3 follows. We estimate it to be very minor in the present experimental conditions, but the collisional state scrambling in the N H 3 beam possibly occurs when the intense molecular beam passes through the hexapole field. Collision-induced transition in (J,K,M) states can change the focusing trajectory to the unfavorable one. Consequently, the falloff in the focusing curves should be interpreted as a contribution from (c) and a negligible contribution from (d). In order to be focused by the hexapole field, a molecule should have both a symmetric-top structure and a relatively large electric dipole moment. Odutola et al. focused the N H 3 dimer using a quadrupole electric field to show a nonzero permanent dipole moment.I4 Fraser et al. measured the dipole moment along the axis with the smallest moment of inertia of the dimer to be = 0.74 D.27 In the present study, the dimer and larger clusters were unfocused by the hexapole field; thus, the structure of the dimer should largely differ from symmetric-top and/or the dimer should have a small electric dipole m ~ r n e n t . * ~ -Structures ~~. providing a very small or zero dipole moment are suggested for the larger clusters, which are consistent with the result of Odutola et al.I4 Figure 4 shows the calculated rotational state distribution of the NH3 monomer at Po = 100 Torr before and after the hexapole state selection at Vo = 10 kV. The distribution function after the ~
~~
(27) (a) Fraser, G.T.; Nelson, Jr., D. D.; Charo, A.; Klemperer, W. J . Chem. Phys. 1985,82,2535. (b) Nelson, Jr., D. D.; Fraser, G. T.; Klemperer, W. J . Chem. Phys. 1985, 83, 6201. (28) Liu, S.-Y.;Dykstra, C. E.; Kolenbrander, K.; Lisy, J. M. J . Chem. Phys. 1986, 85, 2077. (29) Tomoda, S. Chem. Phys. 1986, 110, 431.
J . Phys. Chem. 1988, 92, 5958-5962
5958
state selection shows large populations preferentially in the (J,IKl,lMI) = (333), (222), and (1 11) states while Gandhi and Bernstein have explained that only the (1 11) state is present in their pulsed supersonic beam of NH3.18b This discrepancy is thought to be due to the difference in the expansion conditions. Our focusing curves showed no rotational state resolution, mainly because the broad distributions of the velocity and the incident angle of the beam smear such fine resolution. In principle, it is possible to align the N H 3 molecule by the hexapole field. The average alignment (3p2 - 1)/2 of the distribution shown in Figure 4 is 0.17. The molecules with p = 0 could be removed by the axial beam stop for better alignment.
Using the beam of the aligned N H 3 molecule, two types of reactions could be chosen: one is the reaction along the C3 axis of the molecule, and another is the reaction from the side of the Acknowledgment. The authors gratefully acknowledge financial support received from the Japanese Ministry of Education, Science, and Culture through a Grant-in-Aid Scientific Research 62606004. Registry No. NH,, 7664-41-7. (30) Ohoyama, H.; Kasai, T.; Ohashi, K.; Kuwata, K. Presented at the 18th International Symposium on Free Radicals, Oxford, 1987.
Spectrum, Energy Content, and Relaxation Mechanism of the Photoisomer of the Laser Dye 3,3'-Diethyloxadicarbocyanine Iodide. Laser- Induced Optoacoustic Studies Gabriel M. Bilmes,+Jorge 0. Tocho,* Centro de Investigaciones Opticas ( C l o p ) , (CIC) Casilla de Correo 124, 1900 La Plata, Argentina
and Silvia E. BraSlavsky* Max-Planck-lnstitut fur Strahlenchemie, Stiftstrasse 34-36, 0 - 4 3 3 0 Mulheim a.d. Ruhr, FRG (Received: December 29, 1987; In Final Form: March 23, 1988)
Laser-induced optoacoustic spectroscopy was used to determine various spectroscopic and kinetic parameters of the less thermodynamically stable isomeric form of 3,3'-diethyloxadicarbocyanine iodide (DODCI). The energy of the ground state of the photoisomer was determined to be 1.6 & 0.3 eV larger than that of the thermodynamically stable form. The quantum yields for fluorescence, 4; = 0.06 & 0.02, and for back photoisomerization, q4PN = 0.02 & 0.02, and the absorption cross sections of the photoisomer at different wavelengths were derived under conditions of photostationary populations of ground states of both isomeric forms. The optoacoustic experiments allowed the measurement of the cross sections with an accuracy higher than that of other methods. The results are in agreement with the concept that radiationless deactivation of the excited isomer of DODCI mainly occurs by direct internal conversion.
Introduction The study of the photophysics of cyanine dyes has received great attention in view of their wide use in different fields, from photomedicine to technical uses. In particular these dyes are employed in laser physics as active media or saturable absorbers in modelocking techniques.' For the latter application 3,3'-diethyloxadicarbocyanine iodide (DODCI, Figure 1) is one of the most often used compounds. The photoisomerization of this molecule, starting from the first excited singlet state of the thermally stable form (N), is a well-studied process2 The back reaction of the less-stable ground state of the photoisomer, P to N, is known to be a thermally activated step. However, the spectral and kinetic properties of P are not well studied. They are of great interest because of the role of this isomer in the process of generation of ultrashort pulses and in the determination of the wavelength range of use of this dye.3 At wavelengths where P absorbs, a back photoisomerization of P to N was s ~ g g e s t e d . Using ~ laser-induced optoacoustic spectroscopy (LIOAS) and fluorescence measurements, we have recently estimated that the rate constant for this process is much smaller than the internal conversion from excited to ground state P.5,6 However, assumptions had to be made about the spectroscopic and thermodynamic parameters of P, since they were not known with sufficient accuracy. Adjusting the experimental conditions, LIOAS also allows the determination of calorimetric and spectral properties of, e.g., short-lived species with optical properties similar to those of the parent compound from which they are photopro*To whom correspondence should be addressed. 'Part of the Ph.D. Thesis, Universidad de La Plata, Argentina.
0022-3654/88/2092-5958$01.50/0
d ~ c e d . ~Using this technique, we now present values for the energy content AE of ground-state P, for its fluorescence quantum yield, &, for the efficiency of back-photoisomerization of P to N, c # Jand ~ ~ for , the absorption cross section of the photoisomer, .'(A), from 570 to 655 nm.
Experimental Section DODCI (Eastman Kodak) was used without further purification in ethanol/water (7:3, V / V ) . ~Absorption spectra were measured with a Cary lp spectrophotometer in the Instituto de Investigaciones Fisicoquimicas Tedricas y Aplicadas (INIFTA), La Plata. Absorbances of DODCI as sample and CoCl2 as reference were matched to within f0.005 absorbance units. The basic experimental setup for the LIOAS studies is essentially the same as described in ref 5 (Figure 2 without the continuous-wave (CW) laser), with the following modifications. The excitation source was a homemade nitrogen laser pumped dye laser in Littrow mounting. Rhodamine 560 (Exciton) in methanol was used for the range 540-590 nm, and a solution of rhodamine (1) Sibbett, W.; Taylor, J. R.; Welford, D. IEEE J. Quantum Electron.
1981, QE-17, 500, and references therein.
( 2 ) Dempster, D. N.; Morrow, T.; Rankin, R.; Thompson, G . F. J. Chem. SOC.,Faraday Trans. 2 1972, 68, 1479. ( 3 ) Valdamis, J. A,; Fork, R. L. IEEE J . Quantum Electron. 1986, QE-22, 112. (4) Rullisre, C . Chem. Phys. Lett. 1976, 43, 303. (5) Bilmes, G . M.; Tocho, J. 0.;Braslavsky, S. E. Chem. Phys. Lett. 1987, 134. ..
73s.
(bjscaffardi, L.; Bilmes, G. M.; Schinca, D.; Tocho, J. 0. Chem. Phys. Lett. 1987, 140, 163. ( 7 ) Braslavsky, S . E. Photobiochem. Photobiophys. 1987, Suppl. 13, 83.
0 1988 American Chemical Society
