J . Phys. Chem. 1984,88, 3938-3939
3938 skimmer
tttt
1
ttlt
ISt laser induces
pnd laser induces
selective condensation
(un)selective photodissociation
Figure 3. A schematic representation in which separation of isotopes by selective condensation is significantlyenhanced by (un)selectivevibrational predissociation. The first IR laser selectively excites %F6 in the v3 vibrational mode, thus inhibiting formation of van der Waals clusters containing '*SF6. The 34 sulfur isotope contained in 34SF6Arnclusters which are now significantly heavier than the %F6 molecules concentrate at the center of the free jet. This selective condensation takes place in the collisional part of the gas expansion. A second IR laser then removes the clusters containing 34SF6efficiently from the collision free part of the molecular beam by IR vibrational predissociation, which may or may not be isotopically selective. An encircled 32 represents a %F6 molecule, represents a 34SF6Arzcluster, and a solid circle is an Ar atom.
skimmer and collimator to cause IRVP of 34SF6(m)Ar(n) and the consequent recoil of 34SF6out of the beam, the gas pumped in this region should contain a very high fraction of 34SF6indeed. This separation method in which selective condensation is com-
bined with IRVP is shown schematically in Figure 3. A preliminary experiment along these lines using two cw C 0 2 lasers, one for the selective condensation in the collisional part of the free jet and the second one for IRVP in the collision free region, showed the expected effect. It should be stressed that for the isotopomers of the heavier elements, with very small optical shifts, even unselective IRVP will efficiently separate isotopes in such a multiple laser experiment, i.e., unselective irradiation of a mixture of 'MF, and JMF6(,,&) at wavelengths where all (or almost all) species containing isotopes i and j absorb will cause selective recoil of jMF, out of the beam. The 'MF6 molecules that do absorb cannot undergo IRVP and hence do not recoil strongly out of the beam. I is an inert gas atom. It is not clear at this point whether the combination of selective condensation and IRVP which we have applied successfully to the separation of the isotopomers of sulfur hexafluoride can be applied effectively to the separation of isotopomers of the heavier elements. This would imply that the collisional region of the free jet would have to be used for both (1) cooling of the 'MF6 and JMF, molecules to separate their spectra and (2) selective IR excitation followed by selective condensation.
Acknowledgment. The authors are grateful to the Swiss Fonds National for financial support. Registry No. SF6, 2551-62-4; 32SF6,211 10-12-3;34SF6,31719-72-9; Ar, 7440-37-1;%, 13981-57-2;34S, 13965-97-4.
Sequential Dichromatic-Biphotonic Photochemistry: A Simple Relationship of the Intermediate Lifetime with the Delay between Synthesizing and Photolyzing Pulses G . Ferraudi Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: April 27, 1984)
Sequential biphotonic processes in phthalocyanines, pc,have been investigated in irradiations with two trains of monochromatic pulses. The pulses of one trans, X = 640 nm, were used for the generation of (3m*)Al(pc)C1and the pulses of the other, X = 490 nm and delayed by 0 C 6 d 2 ms with respect to the 640-nm pulses, were used for the photolysis of this state. A direct relationship between the yield of the biphotonic process and the delay time 6 was found to be in agreement with a proposed mechanism for the dichromatic-biphotonic photochemistry.
Introduction
That sequential biphotonic processes take place when metallophthalocyanines are irradiated with high-intensity monochromatic light has been previously e~tab1ished.l~~ In these biphotonic reactions, the lowest-lying 3ir7r*was enough long lived, a condition that allowed it to be the acceptor of the second photon for the formation of the reactive state, eq 1. Similar biphotonic processes
induced by irradiating a t 640 nm with a train of pulses, and at 490 nm with another train of pulses which are delayed by 6 with (1) The abbreviated name, pc, has been used for the phthalocyanine ligand throughout this work. (2) Prasad, D. R.; Ferraudi, G. Inorg. Chem. 1982, 21, 2967. (3) Muralidharan, S.; Ferraudi, G. J . Phys. Chem. 1982, 87, 4877.
0022-3654/84/2088-3938$01.50/0
respect to the first one, were investigated in this work. Such a dichromatic irradiation permitted the finding a direct relationship between the lifetime of the acceptor of the second photon and the delay between pulses. Experimental Section
For the dichromatic-biphotonic irradiations, two flash-pumped dye lasers, Candelas SLL-66A and SLL-200, were synchronously fired in order to obtain a constant delay between the light pulses. The time delay between pulses from each laser, Le., 6 , could then be adjusted up to a maximum of 2 ms with an error, (6 f 0.01) f i s , much smaller than any of the pulse widths (Figure 1). A third beam of monochromatic light traveled along the optical axis and was used for the monitoring of optical transients generated in the laser irradiation. The light of the lasers was directed at sharp angles, namely less than 3 O , with respect to the axis defined by the beam of monitoring light. This optical arrangement ensured a good overlap of the light beams on the sample. A pair of beam splitters and photodiodes allowed the determination of the relative 0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 3939
Letters
0.5570
-.
0.44971 b
I I
} I
SLL-66A
PD
0
$
\
TIME, p~
$
_ _ _. -_ ---- .
BS
M
Figure 1. Optical arrangement used for the dichromatic-biphotoicirradiations with two flash-lamp-pumpeddye lasers, SLL-200 and SLL-66A.
The dashed lines show the light beams from the lasers and the continuous-wave monitoring source, S. In addition this setup uses colimating telescopes, T, monochromator, M, cell holder, CH, mirrors, M, and a light bafle, B. Beam splitters, BS, reflected a small fraction of the lasers light into photodiodes, PD. The insert to the figure shows a typical oscilographic trace used for the control of the experiments. The two spickes separated by a time 6 are the photodiodes response to the laser excitation superimposed with the phototube response, Le., following the decay of the excited state, (3~~*)Al(pc)CL intensities and delay of the laser pulses, and a pulse counter gave the number of pulses arriving at the sample. Conventional and laser flash photolysis were used for the direct observation of reaction intermediates. The photochemical procedures followed in continuous and flash irradiations were described elsewhere.24 The Al(pc)Cl was available from a previous work.4
Results and Discussion Time-resolved experiments in flash photolysis demonstrated that 640-nm irradiations of Al(pc)Cl in deaerated 2-propanol produced the long-lived (3?r?r*)Al(pc)C1. The excited state exhibited a differential spectrum with A,, 490 nm and a half-life 7 330 ps, in good agreement with literature This long life of the excited state determined that Al(pc)Cl underwent facile monochromatic-biphotonic photochemistry in 640-nm irradiations, namely, for photolyses with more than 200 mJ/pulse. Since the monochromatic-biphotonic processes interfered with the detection of the dichromatic-biphotonic photochemistry, the intensities of the 640- and 490-nm pulses were reduced until neither of the monochromatic irradiations were able to induce photochemistry.* Despite the fact that these monochromatic excitations failed to induce photochemical transformations, they were detected in double-irradiated samples, Le., photolyzed first a t 640 nm and at 490 nm after a delay 6 < 300 ps. Moreover, spectroscopic analyses of the irradiated solutions revealed that the photoreactions induced by the dichromatic irradiations or by steady-state UV photolyses (e.g., continuous-wave source of 254-nm light with Zo = 2.0 X 10" einstein/(dm3.s)) were the same; a fact that was in agreement with previous studies on the biphotonic photochemistry of metallophthalocyanines.' Two further observations that are
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(4) Ferraudi, G.; Muralidharan, S. Inorg. Chem. 1983, 22, 1369. (5) Darwent, J. R.; McCubbin, I.; Phillips, D. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 341. (6) Villar, J. G.; Lindqvist, L. C. R . Acad. Sci., Ser. B 1967, 264, 1807. (7) Dhno, T. K.; Kato, S.; Yamada, A.; Tanno, T. J . Phys. Chem. 1983, 87, 115.
(8) Since the monochromatic-biphotonic photochemistry exhibited a quadratic dependence on light intensity,j it was not difficult to find light intensities which gave a negligible rate of biphotonic photochemistry at 640 nm and yet generated sufficiently large concentrations of ('?r?r*)Al(pc)Cl for its photolysis at 490 nm. Moreover, the low extinction coefficients of the ground state, Al(pc)Cl, at 490 nm prevented the monochromatic-biphotonic photochemistry at this wavelength.
Figure 2. Dependence of the rate of dichromatic-biphotonic photochemistry, open circles and right-side ordinate, on the delay 6 between the 490-nm photolyzing pulses and 640-nm synthesizing pulses (the subscripts 6 and 0 in the ordinate label denote 6 and zero delay times). A typical trace, solid circles and left-side ordinate, for the decay of (3mr*)Al(pc)CIfollowed at 490 nm shows the good agreement existent between lifetimes measured in flash photolysis and biphotonic measurements. All these experiments were carried out with solutions of Al(pc)Cl in deaerated 2-propanol.
reported below, Le., the dependence of the product yields on light intensity and on time delay support a mechanism, eq 2, where
products ( 2 )
640-nm laser pulses synthesize (3a?r*)Al(pc)C1with a quantum yield I$,, and 490-nm pulses photolyze this species as in the case of monochromatic-biphotonic photochemistry, eq 1. In these experiments it was possible to regard the (3?r?r*)Al(pc)C1solutions as optically diluted and the width of the laser pulses (width 0.3 ps) very small in comparison to the lifetime of the excited state. The use of these experimental conditions in conventional rate equations gives an expression for the consumption of Al(pc)Cl, eq 3, which shows a linear dependence in the number of photons -A [ Al( pc)Cl] = 2.3031$1~ i 7 7 h i ~ n ~ ~exp(-6 ~ ~ ~ 9/7) ~ N t (3) l in each pulse, n, with i = 640 or 490, and exponential on the delay time Other factors in eq 3 correspond to the extinction coefficient of the triplet state, t, the optical path, I , the number of photolyzing pulses, N , and the quantum yield, qi2,for the product formation in 490-nm photolyses of the ('m*)Al(pc)Cl. A verification of the dependence of the rate of photolysis on the light intensity predicted by eq 3 was carried out by determining the rate of phthalocyanine consumption, Le., -A[Al(pc)Cl]/N, as a function of n640n490in experiments with no delay between 640and 490-nm pulses. The results of these measurements revealed that the rate of complex consumption was linearly dependent on the product n640n490.Moreover, this rate was also exponentially dependent on the delay 6 (Figure 2). A least-squares fit of In(-A[Al(pc)Cl]/N) vs. 6 gave 7 = 320 ps for the (3?r?r*)Al(pc)Cl lifetime, a value that agreed well with those determined n flash photolysis. Such a good agreement between the lifetimes obtained in flash photolysis and biphotonic photolyses shows that the experiments described here can be used for the identification of the acceptor of the second photon in sequential biphotonic photochemistry. Furthermore, it is possible now to devise experiments where, by the proper use of time delays and irradiation wavelengths in biphotonic irradiations, one can arbitrarily modify the photoreactivity of a molecule.
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Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2581 from the Notre Dame Radiation Laboratory. (9) Rate equations were derivated and treated according to Benson, S. W. "The Foundations of Chemical Kinetics"; McGraw-Hill: New York, 1960.
