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Surface picosecond raman gain spectroscopy of a cyanide monolayer on silver
Volume 67, number 2,3
CHEMICAL PHYSICS LETTERS
15 November 1979
SURFACE PICOSECOND RAMAN GAIN SPECTROSCOPY OF A CYANIDE MONOLAYER ON SILVER J.P. HERITAGE, J.G. BERGMAN, A. PINCZUK and J.M. WORLOCK Bell Telephone Laboratories, Holmdel, New Jersey 07733, USA Received 6 August 1979; in final form 12 October 1979
A new surface picosecond Raman gain technique is used to measure the Raman spectrum of a monolayer of CN on silver. A new result, not obtainable from conventional Raman techniques, emerges from this study of the enhanced Raman effect on silver surfaces: the smooth continuum that accompanies enhanced Raman scattering is luminescence, not a Raman effect.
We report a new surface picosecond Raman gain (SPRG) technique which can detect Raman vibrations of a single monolayer of molecules on a metal surface. Using SPRG we have measured a Raman spectrum of a single monolayer of CN on a silver surface. The picosecond temporal resolution, together with the high sensitivity that is possible with SPRG has provided new information not previously obtainable with conventional Raman techniques. SPRG utilizes two time synchronized [ 1 ] picosecond pulse trains tuned to optical frequencies u~ and vs. When Au = u~ - u s = Ur, where ur is the frequency of a Raman active mode, gain is observed at the Stokes frequency u s . The use of continuous trains o f ultrashort optical pulses provides a dramatic increase in sensitivity over that possible with single mode continuous sources [2] because of the high peak intensity of the pulses. The surface picosecond Raman gain technique that we describe is an extension of the picosecond Raman gain technique introduced previously by Heritage [3] for vibrational studies in liquids. Although the sensitivity reported in ref. [3] is impressive, significant improvement was required for studying monolayers. We have achieved the necessary improvement in sensitivity by recognizing that the stability in the amplitude of the picosecond pulses over a few resonator round trip periods is expected to be excellent. In fact, by modulation of the pump beam and synchronous detection of the probe beam at 10.7
MHz we have achieved an improvement in sensitivity by a factor of ~ 5 0 0 compared to the work of ref. [3]. Using a calibrated modulator, we have measured a routinely obtainable minimum detectable modulation depth o f ~ 1 × 10 - 8 , in ls of integration and 8 mW of laser power. This result, obtained with 5 ps pulse widths, is within a factor of ten of the calculated shot noise limit. This excellent noise performance is obtained with careful attention to laser length detuning and modelocking quality. Levine [4] has confirmed the low noise properties of our high frequency synchronous detection techniques and reports that the addition of an intracavity etalon permits shot noise limited performance~ We estimate the signal-to-noise ratio expected from a monolayer of benzene to illustrate the remarkable sensitivity of SPRG and to show its potential for detection of unenhanced Raman vibrations. The steady state gain factor may be calculated from the relation [5] g = (167r2c2N/h6O3sn2p)do/d~2.
We use da/d~2 = 7.8 X 10 -30 cm2/sr [6] for the Raman scattering cross section of benzene vapor. The number density, N, of liquid benzene is 8 X 10 -20 cm - 3 , the Raman line width, P, is 2.4 cm -1 [2] and w s = 27rus = 3.1 X 1015 s - 1 . We obtain a gain factor o f g = 2.7 X 10 -3 cm/MW. If we take 4 A as the layer thickness and employ a 100 W pulse focused to a spot diameter of 10/am then the single pass gain is G = 229
Volume 67, number 2,3
15 November 1979
CHEMICAL PHYSICS LETTERS
gll~ = 1.4 × 10 - 8 . This extremely small gain may be detected since the contribution to the noise from laser fluctuations at 10 MHz is comparably small. The signal-to-noise ratio at the shot noise limit may be calculated from the relation [2]
o
SIN = G(2QPs/hWsB) 1/2 ;
Z CI
we take Q = 0.7, the detector quantum efficiency; Ps = 40 mW tile average power of the Stokes beam, and B = 0.01 s-1 , the detection bandwidth. Here we take the rms value for G. We obtain SIN = 28 for our idealized benzene monolayer. Conventional Raman scattering from a surface monolayer is very difficult since high power is required and luminescence can dominate the Raman signal. We emphasize that monolayer sensitivity is obtained with low energy, non~ damaging pulses while rejecting potential luminescence background. We have neglected in this estimate local field corrections and potential contributions from substrate heating. Levine [7] has calculated a local field correction and orientation effects and estimated the temperature rise on a variety of substrates. One way to separate the background due to heating from the enhanced Raman gain is demonstrated in this paper. We now turn to our experimental investigation of the surface enhanced Raman effect with SPRG. Several molecular species have been shown to exhibit an enhanced Raman effect on the silver surface [8]. Most experiments [9] have been done with samples prepared and studied in an electrochemical cell under potentiostatic control. Two exceptions are CO and CN: Carbon monoxide has displayed an enhanced Raman cross section when adsorbed on silver at low temperature in vacuum [I0]. Cyanide adsorbed on silver in solution has been shown to give strong Raman signals after the sample is rinsed and dried [11,12]. In general, in these experimen~ts one observes significant enhancements (104-106) of the Raman cross sections of well characterized molecular modes, along with certain low frequency vibrations associated with adsorbate-metal motion, and a broad, nearly structureless continuum extending from 300 cm -1 to nearly 4000 cm -1 . We display in fig. 1 a conventional Raman spectrum of cyanide on silver, obtained in air. In-air spectra display an additional band near 1600 cm -1 which has been assigned to carbonate 230
t
1ooo
t 2000
I 3000
v R (cm-I)
Fig. 1. Spontaneous Raman spectrum, obtained in air, of a monolayer of cyanide adsorbed on silver. The feature at 2145 cm -1 is the enhanced CN vibration.
contamination [ 11 ]. The origin of the smooth continuum has not been determined. Given its structureless character over a large frequency range, the techniques used in cw spontaneous Raman spectroscopy are unable to distinguish it from luminescence. The experimental arrangement employed for the investigation of the surface enhanced Raman effect is depicted in fig. 2. Two cw dye lasers (rhodamine 6G and rhodamine 101) are mode-locked by periodic gain modulation impressed by the output of a mode-locked argon ion laser. In this experiment three-plate birefringent tuners limit the pulse widths to ~8 ps and a typical cross-correlation width of 10 ps is obtained. PRELOCK IN AMPLIFIER
L
DELAY I ~ " ~ I I I I
DICHROIC MIRROR h U
FROM LASERS
¢ ; ~ " ~ SILVER ~ SURFACE
E.O. MODULATOR
Fig. 2. Experimental arrangement. Two continuous trains of picosecond pulses are generated by a pair of synchronously mode-locked dye lasers. The optical beat frequency between the lasers is controlled by tuning one laser's wavelength.
Volume 67, number 2,3
CHEMICAL PHYSICS LETTERS
The estimated peak pump power at the sample is 75 W. The lasers are tuned near 5680 A (pump) and 6468 A (probe), corresponding to the Raman frequency shift near 2145 cm -1 . A variable delay line scans the delay between the pump beam and the probe beam. This scan gives time resolved information for various laser frequency detunings. With the delay fixed for optimal temporal overlap, we obtain Raman spectra by rotating a birefringent tuner with a precision drive. Data are accumulated in a multichannel analyzer that is synchronized with either the delay line or the tuner. The collinear pair of beams is focused (focal length = 3 cm) on the surface and is incident at 65 ° from the surface normal with " p " polarization. The reflected beams are collimated and separated by a prism array and the probe beam then falls on a silicon photodiode. The pump beam is modulated at 10.7 MHz and the gain is synchronously detected with a high-frequency lock-in amplifier. A monolayer of cyanide on a silver surface was prepared with the technique described by Bergman et al. [12]. In fig. 3a, trace 1 we present a time resolved scan of A/, the increase in probe beam intensity, obtained with f'Lxed optical beat frequency t ; r = 2145 cm -1 corresponding to the CN vibration peak. One observes an asymmetric profile, peaked near zero delay with a nonzero shoulder for positive delay (pump precedes probe). This profile is the superposition of two components. One is the true Raman gain, which closely follows the pulse probe cross-correlation, peaking at At = 0. The other is a longer-lived change in the apparent reflectivity AR of the surface. Three empirical characteristics of AR are: (1) AR does not require the presence of cyanide, (2) AR looks like the integral of the pump pulse intensity profile, (3) AR decays away in time long compared to the pulse width. These characteristics are displayed in fig. 3a trace 2, which shows AR for a silver surface subjected to a similar electrochemical anodization, but without cyanide in agreement with the first point. The second point is supported by the close agreement o f the AR profile and trace 3 in fig. 3a, which is a normalized integral of the measured cross-correlation. Finally, the AR prol~tle was found to decay in approximately 200 ps with time resolved scans for longer delay. The origin of the observed AR has two possible
15 November 1979
_
/~
<
,.";;" ,:', "t'~'
-,--,n~-~ -20
- -
"""" ~:-"-'2--"-
I
z J
0
20 DELAY (PSEC)
[/ ~
I
i
I
i
I
40
I
60
t.... (b)
i' -10
ta) __.__.__
....... 0
10
20
30
40
DELAY (PSEC)
Fig. 3. (a) Trace 1: measured change in probe beam average power as a function of time delay for cyanide on silver in air. Positive delay means that the pump pulse precedes the probe (Stokes shifted) pulse. Trace 2: measured apparent reflectivity change AR for an electrochemically anodized surface as in trace 1, but without cyanide. Trace 3: normalized integral of a measured cross-correlation. The difference between trace 1 and 3 is the pure Raman contribution at 2145 cm-1 . (b) Trace 1: Raman gain temporal scan with Av = 2145 cm-1 . Trace 2: temporal scan with AV= 2000 cm-1 . A normalized integral of a measured cross-correlation has been subtracted from trace 1 and trace 2, leaving only the pure Raman gain contribution. Raman gain is absent in trace 2. interpretations. The reflectivity may increase as a result o f heating of the surface by the pump pulse, or absorption may decrease through pump-induced population changes. Either mechanism will give a signal which rises like the integral of the pump pulse and then decays with an independent time constant. In either case we can produce the true Raman gain profile by subtraction of a normalized integral of a measured cross-correlation from the experimental trace. We have obtained the Raman gain spectrum o f cyanide on silver in air and display the spectrum in fig. 4. The AR contribution has been subtracted from the measured profile, which shows the CN band at 2145 cm - 1 in agreement with the conventional Raman spectrum. Note that the continuum is absent from the Raman gain spectrum. We now turn our attention to the smooth continuum that extends from ~ 2 0 0 cm - 1 to > 3 0 0 0 231
Volume 67, number 2,3
CHEMICAL PHYSICS LETTERS o
o
o o
o
z
8,~
o °
o
o
o
o Ooo o
0
2045
2105
2165 Vr(cm"1)
2225 o
Fig. 4. Raman gain spectrum, taken in air, around AV= 2145 cm -l , of a cyanide monolayer adsorbed on silver. cm - 1 (fig. 1). We have obtained time resolved traces, one at Av = 2145 cm - 1 (fig. 3b, trace 1) and the second at Av = 2000 cm - 1 on the same site in an effort to determine the nature of the continuum. The integral o f the measured cross-correlation (fig. 3b, trace 2) has been subtracted from the measured profile as discussed above, leaving only the Raman gain. The Raman gain component at Av = 2145 cm - 1 displays the expected profile. Off resonance the gain disappears. The fact that there is no gain o f f resonance proves, without need of further discussion, that the origin of this part of the continuum is luminescence. Taking into account the uncertainty introduced by noise and the measured peak Raman gain at 2145 cm - 1 , we find that a Raman contribution of 5% of the peak could easily be detected in the continuum. Since the peak-to-continuum ratio observed in spontaneous spectra on our in-air samples was typically 5-to-l, we conclude that at no more than 25% of the continuum strength can be Raman effect. Even though the absence of a Raman effect in the continuum has so far been verified only near 2000 cm - 1 , we can anticipate that this result will remain true throughout the featureless region o f the continuum. There may, however, be weak Raman structure added to the continuum, especially when large multimode molecules are adsorbed on the surface. The question for example, of the origin o f the band of structure that lies near 1600 c m - 1 , remains open. The sharp features in this region may be o f Raman 232
15 November 1979
origin, but the adsorbate is not yet positively identified. The low frequency region below about 200 c m - 1, that rises steeply as Av approaches zero is not considered to be part of the continuum and might have a Raman contribution. The excitation [13] and subsequent radiative decay o f surface roughness-induced h o l e - e l e c t r o n pair excitations may account for the luminescence continuum. A complete understanding o f the surface enhanced Raman effect will probably include, as well, a detailed description of the origin of the luminescence continuum. Our contribution, presented in this paper, has been to develop and demonstrate a sensitive, picosecond Raman gain technique and to obtain the first Raman gain spectrum of a monolayer. Additionally, we have obtained the new result, not obtainable with conventional spectroscopic techniques, that the continuum associated with surface enhanced Raman scattering must be luminescence. We gratefully acknowledge helpful conversations with C.V. Shank. We also thank B. Levine and D. Zwemer for conversations and D. Taylor, R. Ferrara and D. Sinatra for technical assistance. References [1] R.K. Jain and J.P. Heritage, Appl. Phys. Letters 32 (1978) 41. [2] A. Owyoung, Opt. Commun. 3 (1977) 23; A. Owyoung and E.D. Jones, Opt. Letters 1 (1977) 152; A. Owyoung, IEEE J. Quantum Electron. QE 14 (1978). [3] J.P. Heritage, Appl. Phys. Letters 34 (1979) 470. [4] B.F. Levine and C. Bethea, IEEE J. Quantum Electron., to be published. [5] M. Maier, Appl. Phys. 11 (1976) 209. [6] J.R. Nestor and E.R. Lippincott, J. Raman Spectry. 1 (1973) 305. [7] B.F. Levine, C.V. Shank and J.P. Heritage, IEEE J. Quantum Electron., to be published. [8] D.L. JeanMarie and R.P. van Duyne, J. Electroanal. Chem. 84 (1977) 1. [9 ] R.P. van Duyne, in: Chemical and biochemical applications of lasers, Vol. 4, ed. C.B. Moore (Academic Press, New York, 1978) ch. 5. [10] T.R. Wood andM.V. Klein, to be published. [11] A. Otto, Surface Sci. 75 (1978) 392. [12] J.G. Bergman, J.P. Heritage, A. Pinczuk, J.M. Worlock and J.H. McFee, to be published. [13] E. Burstein, Y.S. Chen, C.Y. Chen, S. Lundquist and, E. Tosatti, Solid State Commun. 29 (1979) 567.
CHEMICAL PHYSICS LETTERS
15 November 1979
SURFACE PICOSECOND RAMAN GAIN SPECTROSCOPY OF A CYANIDE MONOLAYER ON SILVER J.P. HERITAGE, J.G. BERGMAN, A. PINCZUK and J.M. WORLOCK Bell Telephone Laboratories, Holmdel, New Jersey 07733, USA Received 6 August 1979; in final form 12 October 1979
A new surface picosecond Raman gain technique is used to measure the Raman spectrum of a monolayer of CN on silver. A new result, not obtainable from conventional Raman techniques, emerges from this study of the enhanced Raman effect on silver surfaces: the smooth continuum that accompanies enhanced Raman scattering is luminescence, not a Raman effect.
We report a new surface picosecond Raman gain (SPRG) technique which can detect Raman vibrations of a single monolayer of molecules on a metal surface. Using SPRG we have measured a Raman spectrum of a single monolayer of CN on a silver surface. The picosecond temporal resolution, together with the high sensitivity that is possible with SPRG has provided new information not previously obtainable with conventional Raman techniques. SPRG utilizes two time synchronized [ 1 ] picosecond pulse trains tuned to optical frequencies u~ and vs. When Au = u~ - u s = Ur, where ur is the frequency of a Raman active mode, gain is observed at the Stokes frequency u s . The use of continuous trains o f ultrashort optical pulses provides a dramatic increase in sensitivity over that possible with single mode continuous sources [2] because of the high peak intensity of the pulses. The surface picosecond Raman gain technique that we describe is an extension of the picosecond Raman gain technique introduced previously by Heritage [3] for vibrational studies in liquids. Although the sensitivity reported in ref. [3] is impressive, significant improvement was required for studying monolayers. We have achieved the necessary improvement in sensitivity by recognizing that the stability in the amplitude of the picosecond pulses over a few resonator round trip periods is expected to be excellent. In fact, by modulation of the pump beam and synchronous detection of the probe beam at 10.7
MHz we have achieved an improvement in sensitivity by a factor of ~ 5 0 0 compared to the work of ref. [3]. Using a calibrated modulator, we have measured a routinely obtainable minimum detectable modulation depth o f ~ 1 × 10 - 8 , in ls of integration and 8 mW of laser power. This result, obtained with 5 ps pulse widths, is within a factor of ten of the calculated shot noise limit. This excellent noise performance is obtained with careful attention to laser length detuning and modelocking quality. Levine [4] has confirmed the low noise properties of our high frequency synchronous detection techniques and reports that the addition of an intracavity etalon permits shot noise limited performance~ We estimate the signal-to-noise ratio expected from a monolayer of benzene to illustrate the remarkable sensitivity of SPRG and to show its potential for detection of unenhanced Raman vibrations. The steady state gain factor may be calculated from the relation [5] g = (167r2c2N/h6O3sn2p)do/d~2.
We use da/d~2 = 7.8 X 10 -30 cm2/sr [6] for the Raman scattering cross section of benzene vapor. The number density, N, of liquid benzene is 8 X 10 -20 cm - 3 , the Raman line width, P, is 2.4 cm -1 [2] and w s = 27rus = 3.1 X 1015 s - 1 . We obtain a gain factor o f g = 2.7 X 10 -3 cm/MW. If we take 4 A as the layer thickness and employ a 100 W pulse focused to a spot diameter of 10/am then the single pass gain is G = 229
Volume 67, number 2,3
15 November 1979
CHEMICAL PHYSICS LETTERS
gll~ = 1.4 × 10 - 8 . This extremely small gain may be detected since the contribution to the noise from laser fluctuations at 10 MHz is comparably small. The signal-to-noise ratio at the shot noise limit may be calculated from the relation [2]
o
SIN = G(2QPs/hWsB) 1/2 ;
Z CI
we take Q = 0.7, the detector quantum efficiency; Ps = 40 mW tile average power of the Stokes beam, and B = 0.01 s-1 , the detection bandwidth. Here we take the rms value for G. We obtain SIN = 28 for our idealized benzene monolayer. Conventional Raman scattering from a surface monolayer is very difficult since high power is required and luminescence can dominate the Raman signal. We emphasize that monolayer sensitivity is obtained with low energy, non~ damaging pulses while rejecting potential luminescence background. We have neglected in this estimate local field corrections and potential contributions from substrate heating. Levine [7] has calculated a local field correction and orientation effects and estimated the temperature rise on a variety of substrates. One way to separate the background due to heating from the enhanced Raman gain is demonstrated in this paper. We now turn to our experimental investigation of the surface enhanced Raman effect with SPRG. Several molecular species have been shown to exhibit an enhanced Raman effect on the silver surface [8]. Most experiments [9] have been done with samples prepared and studied in an electrochemical cell under potentiostatic control. Two exceptions are CO and CN: Carbon monoxide has displayed an enhanced Raman cross section when adsorbed on silver at low temperature in vacuum [I0]. Cyanide adsorbed on silver in solution has been shown to give strong Raman signals after the sample is rinsed and dried [11,12]. In general, in these experimen~ts one observes significant enhancements (104-106) of the Raman cross sections of well characterized molecular modes, along with certain low frequency vibrations associated with adsorbate-metal motion, and a broad, nearly structureless continuum extending from 300 cm -1 to nearly 4000 cm -1 . We display in fig. 1 a conventional Raman spectrum of cyanide on silver, obtained in air. In-air spectra display an additional band near 1600 cm -1 which has been assigned to carbonate 230
t
1ooo
t 2000
I 3000
v R (cm-I)
Fig. 1. Spontaneous Raman spectrum, obtained in air, of a monolayer of cyanide adsorbed on silver. The feature at 2145 cm -1 is the enhanced CN vibration.
contamination [ 11 ]. The origin of the smooth continuum has not been determined. Given its structureless character over a large frequency range, the techniques used in cw spontaneous Raman spectroscopy are unable to distinguish it from luminescence. The experimental arrangement employed for the investigation of the surface enhanced Raman effect is depicted in fig. 2. Two cw dye lasers (rhodamine 6G and rhodamine 101) are mode-locked by periodic gain modulation impressed by the output of a mode-locked argon ion laser. In this experiment three-plate birefringent tuners limit the pulse widths to ~8 ps and a typical cross-correlation width of 10 ps is obtained. PRELOCK IN AMPLIFIER
L
DELAY I ~ " ~ I I I I
DICHROIC MIRROR h U
FROM LASERS
¢ ; ~ " ~ SILVER ~ SURFACE
E.O. MODULATOR
Fig. 2. Experimental arrangement. Two continuous trains of picosecond pulses are generated by a pair of synchronously mode-locked dye lasers. The optical beat frequency between the lasers is controlled by tuning one laser's wavelength.
Volume 67, number 2,3
CHEMICAL PHYSICS LETTERS
The estimated peak pump power at the sample is 75 W. The lasers are tuned near 5680 A (pump) and 6468 A (probe), corresponding to the Raman frequency shift near 2145 cm -1 . A variable delay line scans the delay between the pump beam and the probe beam. This scan gives time resolved information for various laser frequency detunings. With the delay fixed for optimal temporal overlap, we obtain Raman spectra by rotating a birefringent tuner with a precision drive. Data are accumulated in a multichannel analyzer that is synchronized with either the delay line or the tuner. The collinear pair of beams is focused (focal length = 3 cm) on the surface and is incident at 65 ° from the surface normal with " p " polarization. The reflected beams are collimated and separated by a prism array and the probe beam then falls on a silicon photodiode. The pump beam is modulated at 10.7 MHz and the gain is synchronously detected with a high-frequency lock-in amplifier. A monolayer of cyanide on a silver surface was prepared with the technique described by Bergman et al. [12]. In fig. 3a, trace 1 we present a time resolved scan of A/, the increase in probe beam intensity, obtained with f'Lxed optical beat frequency t ; r = 2145 cm -1 corresponding to the CN vibration peak. One observes an asymmetric profile, peaked near zero delay with a nonzero shoulder for positive delay (pump precedes probe). This profile is the superposition of two components. One is the true Raman gain, which closely follows the pulse probe cross-correlation, peaking at At = 0. The other is a longer-lived change in the apparent reflectivity AR of the surface. Three empirical characteristics of AR are: (1) AR does not require the presence of cyanide, (2) AR looks like the integral of the pump pulse intensity profile, (3) AR decays away in time long compared to the pulse width. These characteristics are displayed in fig. 3a trace 2, which shows AR for a silver surface subjected to a similar electrochemical anodization, but without cyanide in agreement with the first point. The second point is supported by the close agreement o f the AR profile and trace 3 in fig. 3a, which is a normalized integral of the measured cross-correlation. Finally, the AR prol~tle was found to decay in approximately 200 ps with time resolved scans for longer delay. The origin of the observed AR has two possible
15 November 1979
_
/~
<
,.";;" ,:', "t'~'
-,--,n~-~ -20
- -
"""" ~:-"-'2--"-
I
z J
0
20 DELAY (PSEC)
[/ ~
I
i
I
i
I
40
I
60
t.... (b)
i' -10
ta) __.__.__
....... 0
10
20
30
40
DELAY (PSEC)
Fig. 3. (a) Trace 1: measured change in probe beam average power as a function of time delay for cyanide on silver in air. Positive delay means that the pump pulse precedes the probe (Stokes shifted) pulse. Trace 2: measured apparent reflectivity change AR for an electrochemically anodized surface as in trace 1, but without cyanide. Trace 3: normalized integral of a measured cross-correlation. The difference between trace 1 and 3 is the pure Raman contribution at 2145 cm-1 . (b) Trace 1: Raman gain temporal scan with Av = 2145 cm-1 . Trace 2: temporal scan with AV= 2000 cm-1 . A normalized integral of a measured cross-correlation has been subtracted from trace 1 and trace 2, leaving only the pure Raman gain contribution. Raman gain is absent in trace 2. interpretations. The reflectivity may increase as a result o f heating of the surface by the pump pulse, or absorption may decrease through pump-induced population changes. Either mechanism will give a signal which rises like the integral of the pump pulse and then decays with an independent time constant. In either case we can produce the true Raman gain profile by subtraction of a normalized integral of a measured cross-correlation from the experimental trace. We have obtained the Raman gain spectrum o f cyanide on silver in air and display the spectrum in fig. 4. The AR contribution has been subtracted from the measured profile, which shows the CN band at 2145 cm - 1 in agreement with the conventional Raman spectrum. Note that the continuum is absent from the Raman gain spectrum. We now turn our attention to the smooth continuum that extends from ~ 2 0 0 cm - 1 to > 3 0 0 0 231
Volume 67, number 2,3
CHEMICAL PHYSICS LETTERS o
o
o o
o
z
8,~
o °
o
o
o
o Ooo o
0
2045
2105
2165 Vr(cm"1)
2225 o
Fig. 4. Raman gain spectrum, taken in air, around AV= 2145 cm -l , of a cyanide monolayer adsorbed on silver. cm - 1 (fig. 1). We have obtained time resolved traces, one at Av = 2145 cm - 1 (fig. 3b, trace 1) and the second at Av = 2000 cm - 1 on the same site in an effort to determine the nature of the continuum. The integral o f the measured cross-correlation (fig. 3b, trace 2) has been subtracted from the measured profile as discussed above, leaving only the Raman gain. The Raman gain component at Av = 2145 cm - 1 displays the expected profile. Off resonance the gain disappears. The fact that there is no gain o f f resonance proves, without need of further discussion, that the origin of this part of the continuum is luminescence. Taking into account the uncertainty introduced by noise and the measured peak Raman gain at 2145 cm - 1 , we find that a Raman contribution of 5% of the peak could easily be detected in the continuum. Since the peak-to-continuum ratio observed in spontaneous spectra on our in-air samples was typically 5-to-l, we conclude that at no more than 25% of the continuum strength can be Raman effect. Even though the absence of a Raman effect in the continuum has so far been verified only near 2000 cm - 1 , we can anticipate that this result will remain true throughout the featureless region o f the continuum. There may, however, be weak Raman structure added to the continuum, especially when large multimode molecules are adsorbed on the surface. The question for example, of the origin o f the band of structure that lies near 1600 c m - 1 , remains open. The sharp features in this region may be o f Raman 232
15 November 1979
origin, but the adsorbate is not yet positively identified. The low frequency region below about 200 c m - 1, that rises steeply as Av approaches zero is not considered to be part of the continuum and might have a Raman contribution. The excitation [13] and subsequent radiative decay o f surface roughness-induced h o l e - e l e c t r o n pair excitations may account for the luminescence continuum. A complete understanding o f the surface enhanced Raman effect will probably include, as well, a detailed description of the origin of the luminescence continuum. Our contribution, presented in this paper, has been to develop and demonstrate a sensitive, picosecond Raman gain technique and to obtain the first Raman gain spectrum of a monolayer. Additionally, we have obtained the new result, not obtainable with conventional spectroscopic techniques, that the continuum associated with surface enhanced Raman scattering must be luminescence. We gratefully acknowledge helpful conversations with C.V. Shank. We also thank B. Levine and D. Zwemer for conversations and D. Taylor, R. Ferrara and D. Sinatra for technical assistance. References [1] R.K. Jain and J.P. Heritage, Appl. Phys. Letters 32 (1978) 41. [2] A. Owyoung, Opt. Commun. 3 (1977) 23; A. Owyoung and E.D. Jones, Opt. Letters 1 (1977) 152; A. Owyoung, IEEE J. Quantum Electron. QE 14 (1978). [3] J.P. Heritage, Appl. Phys. Letters 34 (1979) 470. [4] B.F. Levine and C. Bethea, IEEE J. Quantum Electron., to be published. [5] M. Maier, Appl. Phys. 11 (1976) 209. [6] J.R. Nestor and E.R. Lippincott, J. Raman Spectry. 1 (1973) 305. [7] B.F. Levine, C.V. Shank and J.P. Heritage, IEEE J. Quantum Electron., to be published. [8] D.L. JeanMarie and R.P. van Duyne, J. Electroanal. Chem. 84 (1977) 1. [9 ] R.P. van Duyne, in: Chemical and biochemical applications of lasers, Vol. 4, ed. C.B. Moore (Academic Press, New York, 1978) ch. 5. [10] T.R. Wood andM.V. Klein, to be published. [11] A. Otto, Surface Sci. 75 (1978) 392. [12] J.G. Bergman, J.P. Heritage, A. Pinczuk, J.M. Worlock and J.H. McFee, to be published. [13] E. Burstein, Y.S. Chen, C.Y. Chen, S. Lundquist and, E. Tosatti, Solid State Commun. 29 (1979) 567.