Long progressions involving frequency intervals of 180 cm^(−1) are observed in the fluoresence of MS for 3327.5 Å excitation. (AIP

Felker, P. M.

Lambert, Wm. R.

Zewail, A. H.

English

Caltech Authors - Main

Downloaded 21 Dec 2005 to 131.215.225.171. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
Letters to the Editor 1603 
TABLE I. Potential energy surface parameters for NN-HF 
and OC-HF. 
Calculational NN-HF OC-HF 
level R 8 (A) De (cm"1) Re (A) De (cm"1) 
DZ/SCF 2. 951 1460 3.122 1450 
DZ+P/SCF 3.081 737 3.169 1031 
TZ+P/SCF 3.186 540 3. 211 748 
TZ'+P/SCF 3.182 541 3.209 745 
DZ+P/SCEP 2.964 1133 3.074 1361 
DZ+P/ACCD 2.958 1188 3.076 1372 
TZ+P/ACCD 3.069 795 3. 098 1066 
607 cm"1 for NN-HF and 987 cm"1 for OC-HF which 
differ by around 100 cm"1 from the TZ +PI ACCD pre-
dictions. 
The seriousness of basis-set deficiencies and neglect 
of electron correlation effects leads to certain unique 
occurrences. For instance, the R6 of the NN-HF com-
plex at the DZ + P/SCF level is within 0. 001 A of the 
spectroscopically inferred vibrationally-averaged 
monomer separation value. That this is not some ex-
ploitable cancellation of neglected effects is immediately 
revealed by the OC-HF results where the same com-
parison gives a huge difference of 0.121 A. Furthermore, 
vibrational averaging we have carried out for these 
systems using a Numerov-Cooley vibrational wave func-
tion17 for a pseudodiatomic model of the complexes 
shows that R8 -R0 differences cover a range of large cor-
rections (0. 02-0. 05 A). Therefore, structures, force 
constants, and one electron properties of these two and 
perhaps other complexes, when determined with 
small basis sets anci/or the neglect of electron cor-
relation effects, are of questionable value. In the forth-
coming report, we shall examine these deficiencies, 
especially as they pertain to the nature of the weak 
attraction that leads to complex formation. 
The support of the Quantum Chemistry Program of the 
National Science Foundation is gratefully acknowledged. 
The calculations were carried out on the Illinois Theo-
retical Chemistry Minicomputer System and with time 
provided by the University of Illinois Research Board. 
1M. R. Keenan, T. K. Minton, A. c. Legon, T. J. Balle, and 
W. H. Flygare, Proc. Natl. Acad. Sci. U.S.A. 77, 5583 (1980); 
A. C. Legon, P. D. Soper, M. R. Keenan, T. K. Minton, 
T. J. Balle, and W. H. Flygare, J. Chern. Phys. 73, 583 
(1980). 
2p, D. Soper, A. C. Legon, and W. H. Flygare, J. Chern. 
Phys. 74, 2138 (1981). 
3A. C. Legon, P. D. Soper, and W. H. Flygare, J. Chern. 
Phys. 74, 4944 (1981). 
4P. D. Soper, A. C. Legon, W. G. Read, and W. H. Flygare, 
J. Chern. Phys. 76, 292 (1982). 
5A. Hinchliffe, Chern. Phys. Lett. 87, 417 (1982); 85, 531 
(1982); Adv. Mol. Relaxation Int. Process. 19, 277 (1981), 
Vol. 21, p. 151 (1981). 
6y, Bouteiller, M. Allavena, and J. M. Leclercq, Chern. Phys. 
Lett. 84, 91 (1981). 
7L. A. Curtiss and J. A. Pople, J. Mol. Spectrosc. 48, 413 
(1973). 
8Re (HF) = 0. 9168 A, R 8 (CO)= 1.1283 A; Re( NN) = 1. 0977 A [K. 
Huber and G. Herzberg, Constants of Diatomic Molecules 
(Van Nostrand, New York, 1979) ]. 
9S. Huzinaga, J. Chern. Phys. 42, 1293 (1965); T. H. Dunning, 
ibid. 53, 2823 (1970). 
1
"'r. H. Dunning, J. Chern. Phys. 55, 716 (1971). 
11The exponents were: a3d(F)=0.9, a 3d(0)=0.8, a 3d(N)=0.77, 
a 3d(C)=0.75, and a 2p(H)=l.O. 
12W. Meyer, J. Chern. Phys. 64, 2901 (1976); C. E. Dykstra, 
H. F. Schaefer, and W. Meyer, J. Chern. Phys. 65, 2740 
(1976). 
1Sc. E. Dykstra, R. A. Chiles, and M. D. Garrett, J. Comp. 
Chern. 2, 266 (1981). 
14R. A. Chiles and C. E. Dykstra, Chern. Phys. Lett. 80, 69 
(1981); S. M. Bachrach, R. A. Chiles, and C. E. Dykstra, J. 
Chern. Phys. 75, 2270 (1981). 
15C. E. Dykstra, Annu. Rev. Phys. Chern. 32, 25 (1981). 
16A. C. Legon, D. J. Millen, and S. C. Rogers, Proc. R. Soc. 
London Ser. A 370, 213 (1980). 
11J. W. Cooley, Math. Comp. 15, 363 (1961); J. K. Cashion, J. 
Chern. Phys. 39, 1872 (1963); D. J. Malik, J. Eccles, and 
D. Secrest, J. Comp. Phys. 38, 157 (1980). 
Picosecond excitation of jet-cooled hydrogen-bonded 
systems: Dispersed fluorescence and time-resolved 
studies of methyl salicylatea> 
P. M. Felker, Wm. A. Lambert, and A. H. Zewailbl 
Anhur Amos Noyes Laboratory of Chemical Physics, '1 California Institute of Technology, Pasadena, 
California 91125 
(Received 12 May 1982; accepted 1 June 1982) 
Picosecond studies of the excited state proton trans-
fer dynamics of molecules in solution1-s and in the gas 
phaset have received considerable attention. However, 
no time-resolved work has been repor.ted on such 
molecules cooled by supersonic jet expansion. Here, 
we communicate new results on the frequency-dispersed 
and picosecond time-resolved fluorescence character-
istics of the blue-emitting form of jet-cooled methyl 
salicylate (MS). These results, when combined with 
the jet excitation spectra, 5 provide new information 
regarding the spectra of the isolated molecule, and its 
dynamics at different excess vibrational energies. 
J. Chem. Phys. 77(3), 1 Aug. 1982 0021-9606/82/151603-03$02.10 © 1982 American Institute of Physics 
Downloaded 21 Dec 2005 to 131.215.225.171. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
1604 Letters to the Editor 
Jet- Cooled Fluorescence Spectra of MS The excitation of MS in the near UV results in a 
A) 
3320 
B) 
3320 
3440 3560 
3620 3920 4220 
Wavelength (.A) 
4520 4820 
FIG. 1. Dispersed fluorescence of jet-cooled methyl salicy-
late excited at 3327.5 A. Conditions were identical for the two 
spectra: Monochromator resolution = 5 A, laser-nozzle dis-
tance ("' x) = 1 mm, pinhole diameter (;D)"" 150 11, 25 psi He, 
nozzle temperature ("' T) ""70•c. Spectra were found to be un-
changed at increased x. Arrows mark the positions of the 1690 
and 3220 em -t bands, and asterisks mark the detection wave-
lengths at which decays were taken with x = 3 mm and mono-
chromator resolution= 20 A. 
10 
Q) 
(.f) 
..._.. 
en 
'0 
X 
Q) 
-0 
0::: 
-o 
Q) 
> 
~ 
Q) 
(.f) 
.D 
0 
7 
6 
5 
4 
3 0 1.2 
2 
/12ns 
0 • 
2.5 3.7 
Time (ns) 
4.9 6.2 
• 
•• 
• 
-.. 
•• 
• 
• 
• 
fluorescence component that exhibits an extreme 
Stokes shift. 6 ' 7 Experimental evidence points to the 
conclusion that this blue emission arises from an 
excited state species, the phenolic hydrogen of which 
has been displaced towards the carbonyl oxygen. 7- 9 
Although much work2- 5' 7- 13 has been done concerning 
this "blue" form, important questions remain. First, 
the proper description of the formation of this species 
is not unambiguously established. Experiments at 
room temperature in solution, 3 and in low-temperature 
neon matrices10 have suggested that the absorption of 
light and the preparation of the blue form occur 
simultaneously. However, the interpretation of these 
experiments must take into account solvent inter-
actions and/or thermal energy effects. Second, the 
nonradiative decay processes are not fully character-
ized. Emission yields and lifetimes in the gas phase4 ' 13 
and in solution2 ' 3 imply the existence of an efficient 
radiationless decay channel at high excess energies in 
the blue species. Again, the presence of solvent, the 
finite temperature of the sample, or both, introduce 
complications into the interpretation of the results. 
Supersonic jet spectroscopy is expected to eliminate 
these complications due to thermal and solvent effects. 
The supersonic cw jet was prepared by expanding 
25-50 psi of helium seeded with vacuum-distilled MS 
(Aldrich, 98%) into an evacuated chamber kept at 10-4 
Torr. The molecules were excited with~ 15 ps pulses 
obtained by frequency doubling the output of a synchro-
nously-pumped, cavity-dumped dye laser. Fluores-
cence was collected with right-angle geometry, passed 
through a monochromator, and detected with a fast 
photomultiplier. Decays were measured by time-
correlated, single-photon counting. 
Jet-cooled spectra of MS for 3327.5 A excitation are 
presented in Fig. 1. A striking feature is the presence 
\60ps. 
• 
• 
• 
FIG. 2. Fluorescence decay rate of 
MS vs excess energy. Experimental 
conditions: Excitation bandwidth 
= o. 5 A, monochromator resolution 
"'40 A, detection wavelength= 4350 A, 
x=2 mm, D""50 11, 50 psi He, T 
=25•c. The inset shows a typical de-
cay and fitted curve corresponding to 
a point at the high energy end of the 
plot • 
0 500 I 000 1500 2000 
Excess Vibrational Energy (cm- 1) 
J. Chem. Phys., Vol. 77, No.3, 1 August 1982 
Downloaded 21 Dec 2005 to 131.215.225.171. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
Letters to the Editor 1605 
of long progressions involving frequency intervals of 
180 cm·t, which are built on 0, 1690, and 3220 cm·1 
shifts from the excitation energy. The fluorescence 
decays of different bands for this excitation energy 
[see Fig. 1(b)] are well fit by single exponentials and 
give identical lifetimes of 12 ± 0. 5 ns. 
As the excitation energy increases, a marked in-
crease in decay rate is observed. 14 As evident from 
Fig. 2, this increase appears to have a threshold at 
-1300 cm·1 of excess vibrational energy (i.e., the 
energy above 3327. 5 A). 
For excitation at 3327.5 A in the jet, three obser-
vations are pertinent to the characterization of the 
emitting state. First, fluorescence lifetimes are 
identical across the fluorescence spectrum. Second, 
there is no apparent rise time to any of these decays. 
Third, the fluorescence progression extends from the 
excitation energy. These results indicate that the dis-
torted excited state is created simultaneously10 with the 
absorption of a photon in the isolated molecule. It is 
likely that the 180 cm·1 interval corresponds to a 
ground-state out-of-plane bending motion of the "ring" 
that includes the intramolecular hydrogen bond. Pro-
gressions of this interval are built on 1690 and 3220 cm·1 
shifts, the ground state carbonyl and 0-H stretches, 15 
respectively. The appearance of these modes would be 
expected given an excited state distortion involving 
phenolic hydrogen displacement toward the carbonyl. 
For higher excitation energies, the marked short-
ening of the lifetime indicates that an efficient intra-
molecular decay process exists. The onset of this 
shortening at X6 """ 3200 A is consistent with the fall-
off of intensity in the jet-cooled excitation spectrum, 5 
which can be linked to a decrease in fluorescence 
quantum yield. Furthermore, the 1300 cm·1 threshold 
(3. 7 kcal/mol) is in excellent agreement with the 
activation energy for the nonradiative rate derived from 
studies in solution. 3 
The marked change in the decay rate (12 ns to 160 ps) 
presented here, in light of recent time-resolved jet 
experiments, 16 ' 17 is suggestive of some pertinent 
considerations regarding MS radiationless decay. In 
particular, it is, perhaps, not entirely accidental that 
the behavior shown in Fig. 2 so closely parallels that 
of !-stilbene. 16 The existence of low frequency torsions 
and bends, which can promote excited state configuration 
change or electronic state coupling, could explain the 
decay behavior of MS and t-stilbene as opposed to the 
more gradual fall-off in lifetime of more rigid mole-
cules such as anthracene. 17 The involvement of such 
motions in MS radiationless decay is further supported 
by the prominence of the 180 cm·1 progression, and by 
the viscosity dependence of the fluorescence lifetime in 
solution. 3 A full account of this work will be published 
later. 
a>This work was supported by a grant from the National Science 
Foundation, No. CHE81-12833. 
b) Alfred P. Sloan Fellow and Camille &Henry Dreyfus Teacher-
Scholar. 
c>contribution No. 6643. 
1(a) W. M. Hetherington, III, R. H. Micheels, andK. B. Eisenthal, 
Chern. Phys. Lett. 66, 230 (1979); (b) s. -Y. Hou, W. M. 
Hetherington, III, G. M. Korenowski, and K. B. Eisenthal, 
ibid. 68, 282 (1979). 
2D. Ford, P. J. Thistlethwaite, and G. J. Woolfe, Chern. 
Phys. Lett. 69, 246 (1980). 
3K. K. Smith and K. J. Kaufmann, J. Phys. Chern. 82, 2286 
(1978); [for relevent studies on other systems see the work 
by Rentzepis' group; P. Barbara, P. Rentzepis, and L. 
Brus, J. Am. Chern. Soc. 102, 2786 (1980)]. 
4R. Lopez-Delgado and S. Lazare, J. Phys. Chan. 85, 763 
(1981). 
5L. A. Heimbrook, J. E. Kenny, B. E. Kohler, and G. W. 
Scott, J. Chern. Phys. 75, 5201 (1981). 
6J. K. Marsh, J. Chern. Soc. 125, 418 (1924). 
7A. Weller, z. Electrochem. 60, 1144 (1956). 
8K. Sandros, Acta Chern. Scand., Ser. A 30, 761 (1976). 
9W. Klopffer and G. Naundorf, J. Lumin. 8, 457 (1974). 
10J. Goodman and L. E. Brus, J. Am. Chern. Soc. 100, 7472 
(1978). 
11w. Klopffer and G. Kaufmann, J. Lumin. 20, 283 (1979). 
12A. u. Acuna, F. Amat-Guerri, J. Catalan, and F. Gonzalez-
Tablas, J. Phys. Chern. 84, 629 (1980). 
13A. u. Acuna, J. Catalan, and F. Toribio, J. Phys. Chern. 
85, 241 (1980). 
14At these high energies, a nozzle temperature dependent de-
cay component of ;:,: 8 ns is observed which all but disappears 
at low temperatures. One possible e;qianation of this com-
ponent, which appears more prominently when carrier gases 
other than He are used, is the presence of MS aggregates in 
the beam. 
15W. W. Morris, J. Assoc. Off. Anal. Chern. 56, 1037 (1973). 
16J. A. Syage, Wm. R. Lambert, P. M. Felker, A. H. Zewail, 
and R. M. Hochstrasser, Chern. Phys. Lett. (in press). 
17Wm. R. Lambert, P. M. Felker, and A. H. Zewail, J. Chern, 
Phys. 75, 5958 (1981); A. Zewail, Wm. Lambert, P. Felker, 
J. Perry, and W. Warren, J. Phys. Chern. 86, 1184 (1982); 
Wm. Lambert, P. Felker, and A. Zewail (in preparation). 
J. Chern. Phys., Vol. 77, No.3, 1 August 1982 


Picosecond excitation of jet-cooled hydrogen-bonded

systems: Dispersed fluorescence and time-resolved

studies of methyl salicylatea

http://authors.library.caltech.edu/32501/1/FELjcp82.pdf

Picosecond excitation of jet-cooled hydrogen-bonded systems: Dispersed fluorescence and time-resolved studies of methyl salicylatea

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main