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Four wave mixing spectroscopy of mixed crystals using three input frequencies
858
Journal of Lumineseence SI & 32 (1984) 858-860 North-F lolland Anisterdarn
FOUR WAVE MIXING SPECTROSCOPY OF MIXED CRYSTALS USING THREE INPUT FREQUENCIES* Jack K. STEEHLER, Dinh C. NGUYEN, and John C. WRIGHT Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706 Unambiguous three input frequency scanning methods are described for pentacene in several host crystals, specifically defining ground or excited state vibrations. Implications for site selective experiments are discussed. INTRODUCTION Theoretical studies have shown that multiresonant nonlinear spectroscopy has possibilities for site selection (selection of a subset of molecules to 2 The selection involves destructive interfergenerate the observed signal).” ences among nonselected sites when the product of two resonant denominators containing damping terms of opposing sign is averaged over the total population distribution.
Specific correlations between inhomogeneous distributions in the
resonant energy levels are required. While doubly resonant cases can give site selection, the ability to achieve triple resonance provides greater flexibility for site selection, and more specific scanning. Two input frequency multiresonant four wave mixing experiments can give double resonances and approach the triply resonant condition, yielding information about vibrational levels in several electronic states.3 Each spectrum contains resonant peaks of both types of vibrations (Figure la). Spectra obtained at different values of the fixed laser frequency identify the electronic state involved, since shifts on an ~ frequency axis occur for only one of the two types of resonance.4 Use of a third input frequency provides full (triple) resonance, allowing selection of any of the three types of molecular resonance. Different combinations of two scanning frequencies specifically access either ground or excited state vibrations. In general, additional molecular states may be resonant with other terms in the expression for these three input frequencies, e.g. two photon resonant terms or terms with the roles of
and cc
3 reversed. However, the triply resonant case discriminates against contributions from less resonant terms unless the concentration of the component involved is significantly *ThiS work was supported by the National Science Foundation under grant CHE83-O6O84. 0022—231 3/84/$O3 .00 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
J.K. Steehier eta!.
I Four ware mixing spectroscopy of mixed crystals
larger than the concentration of the triply resonant component.
859
For host
molecules in this study even singly resonant contributions are observed. RESULTS In Figure 1
,
the higher sensitivity and more specific information content of
four wave mixing spectra using three separately tunable input frequencies is illustrated for 10-6 mole/mole pentacene, at <2K. When both pure electronic and ground state vibrational resonances are established by proper choice of 01 and W2~ scanning 03 and 04 synchronously yields resonances for excited state vibrations specifically (Figure ib). Only the excited state vibration at 747 cm~, the same mode as selected by the 756 cm~ ground state vibration, appears in this simpler, higher signal level spectrum (compared to the two input frequency case, Figure la, where both v and v resonances appear). Similarly, selection of pure electronic and excited state vibrational resonances with 0A provides spectra containing only ground state vibrational fixed 01 and resonances when 02 and 03 are synchronously scanned (Figure lc). Pentacene ground state resonances at 756, 763 and 787 cm1 are present, with the strongest signal corresponding to the 756 cm~mode selected by the fixed excited state resonance at 747 cm~.
The weaker signal at 747 cm~ corresponds
to a degenerate position in the spectrum, where the 2u processes are not resolved.
1_cu2, 203_02. and Observation of a ground state resonance
does not prove the involvement of an intermode coupling term, unless the intensity of that resonance is shown to depend on the degree to which cc4 matches the energy of the selected excited state vibration. Spectra obtained with 04 nonresonant show all pentacene ground state resonances observed in Figureacid lc are greatly proving reduced the compared to the of constant intensity 1 benzoic resonance, occurrence intermode coupling 798 cm in lc. Relative intensities of individual ground state resonances in this type of scan are controlled by the transition dipole moment and damping constant between that ground state vibration and the selected fixed excited state vibration. Spectra have also been obtained using three input frequencies for both CARS (Figure 2) and CSRS in p-terphenyl host,5 a system with four distinct sites. Overlapping resonances from different sites have been resolved by the site selectivity achieved when two selectively resonant levels define the subset of sites that give rise to the signal observed when scanning the third resonance frequency. Figure 2a shows ground state resonances of site 1 only, at 789, 765, 760.5, 759, and 753 cm1, with a degenerate peak at 750 cm~. Figure 2b shows ground state resonances of site 2 only, at 789, 764, 758, and 750 cm1.
J. K. Sue/tier eta!.
860
/ I-our wale isiLving spectroscopy of mixer!
crtstals
The selectivity required for these very similar sites is made possible by the use of three tunable input frequencies. Spectra obtained with two input 5 show resonances of both sites, since only one element of selection frequencies is available versus two elements when three input frequencies are used. The CSRS spectra are also unique because three input frequencies allow the CSRS output energy to be greater than any fluorescence energy, which is a strong interference in multiresonant CSRS using two input lasers.
ikJL~~~~jI ~
:~
____
7~O
770
Ic~’t
80
Figure 1. CARS spectra of pentacene in benzoic acid, rc~=00. a) Two input lasers3, scan ~2 and 04. b) Three input lasers, scan 03 and A’ 0l_02 = 756 cm’ ( cc). c) Three input lasers, scan cc 1 ( vo~. 2 and 0(3~(04 = ~~0O + ~V cm
eóo
no
E,t ~
_
740
Figure 2. Three input laser CARS spectra of pentacene in p-terphenyl, scanning ~ and (03. a) -l ~ (sitel), 0A Woo+ 750(01cm :l~OO = ~ (site (site b) 2), 1).A= ~bo 749 cm = 0(0 (site 2).
REFERENCES 1)
S. A. 3. Druet, 3.-P. Taran, and C. 3. Bordd, 3. Physique 40 (1979) 819.
2)
J.—L. Oudar and V. R. Shen, Phys. Rev. A 22 (1980) 1141.
3)
P. L. Decola, 3. R. Andrews, R. M. Hochstrasser, and H. P. Trommsdorff, 3. Cheni. Phys. 73 (1980) 4695.
4)
R. Bozio, P. L. Decola, and R. M. Hochstrasser, in: Time Resolved Vibrational Spectroscopy, Ed. 6. H. Atkinson (Academic Press, New York, 1983) pp. 335—344.
5)
3. K. Steehler and 3. C. Wright, unpublished results.
Journal of Lumineseence SI & 32 (1984) 858-860 North-F lolland Anisterdarn
FOUR WAVE MIXING SPECTROSCOPY OF MIXED CRYSTALS USING THREE INPUT FREQUENCIES* Jack K. STEEHLER, Dinh C. NGUYEN, and John C. WRIGHT Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706 Unambiguous three input frequency scanning methods are described for pentacene in several host crystals, specifically defining ground or excited state vibrations. Implications for site selective experiments are discussed. INTRODUCTION Theoretical studies have shown that multiresonant nonlinear spectroscopy has possibilities for site selection (selection of a subset of molecules to 2 The selection involves destructive interfergenerate the observed signal).” ences among nonselected sites when the product of two resonant denominators containing damping terms of opposing sign is averaged over the total population distribution.
Specific correlations between inhomogeneous distributions in the
resonant energy levels are required. While doubly resonant cases can give site selection, the ability to achieve triple resonance provides greater flexibility for site selection, and more specific scanning. Two input frequency multiresonant four wave mixing experiments can give double resonances and approach the triply resonant condition, yielding information about vibrational levels in several electronic states.3 Each spectrum contains resonant peaks of both types of vibrations (Figure la). Spectra obtained at different values of the fixed laser frequency identify the electronic state involved, since shifts on an ~ frequency axis occur for only one of the two types of resonance.4 Use of a third input frequency provides full (triple) resonance, allowing selection of any of the three types of molecular resonance. Different combinations of two scanning frequencies specifically access either ground or excited state vibrations. In general, additional molecular states may be resonant with other terms in the expression for these three input frequencies, e.g. two photon resonant terms or terms with the roles of
and cc
3 reversed. However, the triply resonant case discriminates against contributions from less resonant terms unless the concentration of the component involved is significantly *ThiS work was supported by the National Science Foundation under grant CHE83-O6O84. 0022—231 3/84/$O3 .00 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
J.K. Steehier eta!.
I Four ware mixing spectroscopy of mixed crystals
larger than the concentration of the triply resonant component.
859
For host
molecules in this study even singly resonant contributions are observed. RESULTS In Figure 1
,
the higher sensitivity and more specific information content of
four wave mixing spectra using three separately tunable input frequencies is illustrated for 10-6 mole/mole pentacene, at <2K. When both pure electronic and ground state vibrational resonances are established by proper choice of 01 and W2~ scanning 03 and 04 synchronously yields resonances for excited state vibrations specifically (Figure ib). Only the excited state vibration at 747 cm~, the same mode as selected by the 756 cm~ ground state vibration, appears in this simpler, higher signal level spectrum (compared to the two input frequency case, Figure la, where both v and v resonances appear). Similarly, selection of pure electronic and excited state vibrational resonances with 0A provides spectra containing only ground state vibrational fixed 01 and resonances when 02 and 03 are synchronously scanned (Figure lc). Pentacene ground state resonances at 756, 763 and 787 cm1 are present, with the strongest signal corresponding to the 756 cm~mode selected by the fixed excited state resonance at 747 cm~.
The weaker signal at 747 cm~ corresponds
to a degenerate position in the spectrum, where the 2u processes are not resolved.
1_cu2, 203_02. and Observation of a ground state resonance
does not prove the involvement of an intermode coupling term, unless the intensity of that resonance is shown to depend on the degree to which cc4 matches the energy of the selected excited state vibration. Spectra obtained with 04 nonresonant show all pentacene ground state resonances observed in Figureacid lc are greatly proving reduced the compared to the of constant intensity 1 benzoic resonance, occurrence intermode coupling 798 cm in lc. Relative intensities of individual ground state resonances in this type of scan are controlled by the transition dipole moment and damping constant between that ground state vibration and the selected fixed excited state vibration. Spectra have also been obtained using three input frequencies for both CARS (Figure 2) and CSRS in p-terphenyl host,5 a system with four distinct sites. Overlapping resonances from different sites have been resolved by the site selectivity achieved when two selectively resonant levels define the subset of sites that give rise to the signal observed when scanning the third resonance frequency. Figure 2a shows ground state resonances of site 1 only, at 789, 765, 760.5, 759, and 753 cm1, with a degenerate peak at 750 cm~. Figure 2b shows ground state resonances of site 2 only, at 789, 764, 758, and 750 cm1.
J. K. Sue/tier eta!.
860
/ I-our wale isiLving spectroscopy of mixer!
crtstals
The selectivity required for these very similar sites is made possible by the use of three tunable input frequencies. Spectra obtained with two input 5 show resonances of both sites, since only one element of selection frequencies is available versus two elements when three input frequencies are used. The CSRS spectra are also unique because three input frequencies allow the CSRS output energy to be greater than any fluorescence energy, which is a strong interference in multiresonant CSRS using two input lasers.
ikJL~~~~jI ~
:~
____
7~O
770
Ic~’t
80
Figure 1. CARS spectra of pentacene in benzoic acid, rc~=00. a) Two input lasers3, scan ~2 and 04. b) Three input lasers, scan 03 and A’ 0l_02 = 756 cm’ ( cc). c) Three input lasers, scan cc 1 ( vo~. 2 and 0(3~(04 = ~~0O + ~V cm
eóo
no
E,t ~
_
740
Figure 2. Three input laser CARS spectra of pentacene in p-terphenyl, scanning ~ and (03. a) -l ~ (sitel), 0A Woo+ 750(01cm :l~OO = ~ (site (site b) 2), 1).A= ~bo 749 cm = 0(0 (site 2).
REFERENCES 1)
S. A. 3. Druet, 3.-P. Taran, and C. 3. Bordd, 3. Physique 40 (1979) 819.
2)
J.—L. Oudar and V. R. Shen, Phys. Rev. A 22 (1980) 1141.
3)
P. L. Decola, 3. R. Andrews, R. M. Hochstrasser, and H. P. Trommsdorff, 3. Cheni. Phys. 73 (1980) 4695.
4)
R. Bozio, P. L. Decola, and R. M. Hochstrasser, in: Time Resolved Vibrational Spectroscopy, Ed. 6. H. Atkinson (Academic Press, New York, 1983) pp. 335—344.
5)
3. K. Steehler and 3. C. Wright, unpublished results.