- Email: [email protected]
Evanescent wave fluorescence spectra of Na atoms
1 May 1997
OPTICS COMMUNICATIONS Optics Communications 137 (1997) 249-253
ELSEVIER
Evanescent wave fluorescence spectra of Na atoms V.G. Bordo ‘, C. Henkel, A. Lindinger, H.-G.Rubahn
*
Received 25 April 1996: revised 20 November 1996; accepted 30 December 1996
Abstract Laser induced fluorescence of Na atoms excited inside the evanescent wave of a glass prism is spectrally resolved for the first time, showing a pronounced dependence of linewidth on the surface temperature in the temperature range of 293 K to 435 K. We are able to reproduce the measured fluorescence lines quantitatively by use of the perturbative density matrix theory. The good agreement between theory and experiment emphasizes the fact that the response of the gas to the evanescent field arises mainly from the atoms desorbing from the surface and is of nonlocal nature. PACS: 32.50. + d; 32.70.J~; 42.50.-p
1. Introduction Resonant optics at a gas-solid interface has attracted growing interest during the last years [l-l I]. The excitation of gas atoms in the close vicinity of a surface results in a transient polarization behaviour of the atoms just having undergone a collision with the surface that leads to a nonlocal connection between the electric field and the induced polarization in the gas [ 121. Experimental evidence for such a behaviour was demonstrated via selective reflection spectroscopy from a glass-vapor interface, both in near-normal incidence [ 12.131 and total internal reflection geometries [ 14,15]. It was observed as a sub-Doppler structure in the reflection coefficient. The fluorescence spectrum excited by spectrally scanning a normally incident laser beam has also been studied [ 161.Another way to investigate these effects is to spectrally resolve the fluorescence, which has been excited by evanescent wave (EW) laser light. This approach facilitates direct comparison with theoretical predictions, which in turn show that the relevant spectra will display features connected with the atom-surface interaction [6-g]. We thus use the latter method in the present paper for the first time to demon-
* Corresponding author. E-mail: rubahnemsfdl .dnet.gwdg.de. ’ Permanent address: Institute of General Physics. Russian Academy of Sciences. 117942 Moscow. Russian Federation.
strate evidence for a nonlocal optical response of Na vapor atoms, which are resonantly excited by an evanescent wave at a glass prism surface. The measured fluorescence line shapes show good agreement with the theoretically predicted spectra and are described by the surface temperature. This means that the fluorescence signal arises mainly from the atoms thermally equilibrated at the surface and then desorbed into the gas phase. To our knowledge this is the first experiment allowing to discriminate the contribution of desorbing atoms in one-photon excitation, while in selective reflection this is possible only in three-level cascade scheme [5].
2. Theory The theory of resonance fluorescence from a gas of two-level atoms excited by an EW at a gas-solid interface was developed in a number of papers that deal with different regimes of the atom-EW interaction [6-g]. In order to compare the present experimental results with the theory two remarks are necessary. First. for a laser power of 400 mW the Na 3S,,, + 3P,,, transition is strongly saturated. Nevertheless perturbation theory with respect to the EW amplitude can still be applied for a quantitative description of the fluorescence line shape as long as the Rabi frequency for the relevant transition is considerably
0030.4018/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. P/I s0030-4018(97)00010-2
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V.G. Bo& et trl./ Optics Cnrltrlllrrli(.L1tiOtl.~ 137 ( lYY7J24Y-253
smaller compared to the Doppler width. In this case. which is fulfilled for the present experiment. one can use the results obtained for a weak EW field [6,7] keeping in mind that this treatment does not give valid line irzrensitks. The second remark concerns the fact that the 3S,,2 + 3P,,, transition cannot be considered as a two-level system because of the hyperfine splitting of the ground state. The hyperfine splitting of the excited state is not resolved. However. as the laser is tuned in resonance with one of the 3s ,,? sublevels (F = I?), the radiation intensity arising from optical pumping to the other sublevel (F = I) is significantly less than the intensity of the stimulated emission at the resonant transition and can thus be neglected. The fluorescence spectrum might be determined by the Fourier transform of the atomic dipole moment two-time correlation function. The latter one is calculated here within a density matrix formalism. The master equation for the atomic density matrix has the form of optical Bloch equations containing the derivatives relative to the coordinate along the normal to the interface. To solve this set of differential equations one must specify separate boundary conditions for atoms moving towards and away from the surface. The solution for the atoms departing from the surface is determined by the sticking probability of a gas atom at the surface, S. We assume S = I. which is a reasonable assumption for Na atoms which hit a dielectric surface at room temperature [ 171. The resulting fluorescence spectrum can be written in the form [6]
g(W’)=g,(w’)+g:(w’)+g:,~w’).
(1)
T(L’)=y~KL’,+i(d+cl’.,).
- K 1‘.
’
(KL’;)2+(Or--W+p.ZJ)2
/+T 7’ where j--
6
(2)
(3)
r) ’
and
I X
(8)
L’7+
3. Comparison
KI‘; + i( w’ - w +p
(6)
P(s) = I’(L’,‘(‘,._ (..I, d = w,, - w, y = T- ’ with T the transverse relaxation ;ime of the atomic transition, K = fi- ’ with 6 the exponential decay length of the EW and p = q - k. We have assumed that the EW propagates along the .V axis and has wave vector q, and that the observed fluorescence light has wave vector k and frequency w’. The fluorescence spectral density has three contributions. Firstly. a contribution from atoms which approach the surface. g;. Secondly, a contribution from atoms which desorb from the surface. a,‘. Both contributions are centered near the EW frequency, w. Finally, there is a contribution from desorbing atoms, which give rise to fluorescence light, which is centered near the atomic transition frequency. w,,. The latter term is caused by the trmsiertt behaviour of the desorbing atoms and is thus correlated with the nonlocal response of the gas atoms near the surface [I’?]. The relative weight of this contribution can be estimated by the ratio
is the mean time-of-flight across the EW field. Under our experimental conditions, 6 = 245 nm. ~7 = 500 m/s, T = 33 ns, we find R = 67. Therefore the third, nonlocal term dominates the experimentally determined fluorescence spectrum.
where
X
is a Maxwellian distribution function with 1‘7_ the most probable velocity of atoms approaching to (departing from) the surface.
(4)
y+i(w’-wO-k.Z’)
Here, we have introduced the following (~‘,,L.,.,v_) is the atomic velocity,
notations:
z’ =
with experiment
The experimental set up (Fig. I) consists of a glass prism. which is mounted inside a vacuum chamber ( pc,I 10m7 mbar) on a heatable manipulator. A flux of Na atoms from a resistively heated oven CT,,,, = 428 K) reaches the prism surface under an angle of 45” with respect to the surface normal. The light from an Art laser pumped single mode ring dye laser (CR 699-21) irradiates the prism via a Brewster angle window at an angle larger than the angle of total internal reflection. It leaves the vacuum apparatus through another Brewster angle window. The most important source of stray light stems from light which is scattered from defects on the prism surface. This background light intensity has been measured to be significantly less than 10-j of the incoming light intensity [ 181. The dye laser is set at a fixed frequency of 16973.33(l) cm-‘. where the error results solely from the spectral
V.G. Bordo rt al. / Optics Conmunioatioru
evanescent
\/
137
f 1997)
249-253
wave
photomultiplier
Fig. 1. Scheme of the experiment: 3S-3P resonance fluorescence of Na atoms (h us,,, ) is excited (hr;,,) within the evanescent wave of a glass prism and is spectrally resolved using an etalon with a FSR of 7.5 GHz and a finesse of 100. The UHV machine is decoupled from the detector using an optical fiber.
resolution of the wavemeter. The FWHM of the laser is 6.7 X 1O-5 cm-’ with a drift of far less than 3 X IO-” cm-’ during an etalon scan. The laser light is used to excite Na atoms in the EW above the surface via the 3S, ,?( F = 2) + 3P,,,(F) transition. The resulting fluorescence light is observed via a collection lens at an angle of 45” with respect to the surface normal and is coupled into a multimode optical fiber. The light ouput of the fiber is directed into a Fabry-Perot etalon with a FSR of 7.5 GHz, which is scanned via a mirror mounted on a piezoelectric crystal. The transmission of the etalon is recorded by a photomultiplier and photon counting electronics. Accurate alignment of the etalon results both in a high finesse (F = 100 in the present case) and maximum signal-to-noise ratio. This alignment is achieved by a decoupling vacuum apparatus and detection optics via an optical fiber and prealigning the etalon using single mode laser light. In Fig. 2 a typical spectrum obtained from fluorescing atoms in the evanescent wave above the prism is pre-
-4 -3 -2 -1 0
1 2 3 4 5 6
Detuning [GHz] Fig. 2. Spectrally resolved fluorescence of Na atoms (3S,,? (F=?) + 3P,,> (F)). excited with a single mode dye laser at 16973.33( 1) cm _’ in the evanescent wave of a glass prism CT = 430 K. P,_ = 400 mW).
I -3000
I
I
I
-2000
-1000
0
I
Detuning (MHz) Fig. 3. Same as Fig. 2. but for surface temperatures of 293 K (a) and 435 K (b). The solid lines result from the theoretical approach as described in the text.
sented. The surface temperature was 430 K. The sharp line results from light which is scattered at defects on the surface of the prism. The broad line on the left-hand side of this peak is the fluorescence spectrum from desorbing Na atoms. As shown in Fig. 3. the width of this latter line depends on the surface temperature. Note that in contrast to usual gas phase spectra the fluorescence spectrum observed via evanescent wave excitation consists of only ~fre tluorescence line. To ensure that we are observing mainly atoms excited inside the evanescent wave and not in the gas phase the exciting laser beam was aligned to propagate parallel to the prism and the fluorescence light was spectrally resolved. Changing the temperature of the prism, we did not observe any change in the fluorescence line shapes, in contrast to the spectra observed in the evanescent wave configuration. In the same instance changing the temperature of the Na oven and thus the flux of incoming Na atoms did not change the spectra, which were recorded in the EW configuration. The experimental spectra are compared in Figs. 3a and 3b withfluorescence spectra calculated from Eq. (4) for different prism temperatures. Theoretical and experimental fluorescence intensities have been normalized at the position of the fluorescence peak. The theoretical value of the transition frequency is 16973.34 cm-’ in both cases, which is within error bars with the experimentally used
V.G. Bordo er al. / Optics Communicntions
252
I
I
280
I
320
I
360
I
400
440
Surface temperature [K] Fig. 4. Measured width (FWHM) of the spectrally resolved fluorescence of Na atoms in the evanescent wave as a function of temperature of the prism surface. The solid line results from numerical calculations using Eq. (4) of the text.
value. Obviously the experimentally determined line shapes are well described by the theoretical profiles obtained for nonlocal contribution for both low and high prism temperatures. In Fig. 4 the measured half widths of the fluorescence lines are plotted as a function of prism surface temperature. Within error bars both the absolute magnitude of half width as well as the increase with increasing temperature are well represented by the theoretical approach using Eq. (4). It should be stressed that we have essentially nonequilibriumconditions at the surface. The atomic flux incident on the surface from the gas phase is dictated by the vapor pressure inside the Na oven (= IO-’ Torrl. resulting in a mean free path A of the order of a few cm. The linear dimensions of the flux of Na atoms are much smaller than A. This means that the atoms collide with the surface much more frequently than with each other. Under such (“collision-free”) conditions thermodynamical equilibrium cannot be established for the atoms near the surface [19]. Therefore, there are two atomic fluxes in the vicinity of the surface: (i) incident on the surface and determined by the oven temperature, and (ii) desorbing from the surface and determined by the surface temperature. Thus the atomic flux desorbing from the surface gives the only contribution to the fluorescence intensity which can be associated with the prism temperature.
4. Conclusions In this paper, we have investigated the fluorescence spectrum of Na atoms excited by an evanescent wave at a glass prism surface. The exponential decay length of the EW was significantly smaller than the flight path of the Na atoms during the relaxation time. This condition leads to rronlocality of the optical response of the gas atoms near the surface. As a direct consequence the fluorescence
137
ClYY7J 249-253
spectrum contains a component arising from atoms that have been desorbed from the surface. This component is centered near the atomic transition frequency. In our case, this line dominates in magnitude. Its appearance is a novel manifestation of the nonlocal optical response of the gas in the close vicinity of a surface while in selective reflection nonlocality leads to a sub-Doppler structure in the reflection coefficient. Experimentally, this prediction has been checked by spectrally resolving the fluorescence of Na atoms excited within the EW of the prism. Using thermodynamically nonequilibrium conditions at the surface (different oven and prism temperatures) and “collision-free” atomic flux, we have demonstrated that the fluorescence line shape is determined by the surface temperature and does not depend on the temperature of the incoming flux. Thus the fluorescence indeed arises mainly from the atoms leaving the surface. This method opens new opportunities to study desorption processes from the surface into the gas atmosphere. In particular, the integral intensity of the fluorescence line is determined by the atomic tlux desorbing from the surface. Thus the temperature dependence of this signal will provide information concerning the desorption rate and hence, the heat of adsorption. Measurements of such kind are in progress.
Acknowledgements We are grateful to the Deutsche Forschungsgemeinschaft for financial support. which enabled the joint work presented here. We also thank J.P. Toennies for encouragement and generous support and the European Community (network “Laser controlled dynamics of molecular processes and applications”) for partial financial support. One of us (V.G.B.) is grateful to the Max-Planck-Institut fur Stromungsforschung for hospitality.
References
[II M. Chevrollier.
M. Fichet. M. Oria, G. Rahmat. D. Bloch and M. Ducloy. J. Phys. II (Paris) 2 (19921 63 I. El A. Weis. V.A. Sautenkov and T.W. Hansch. Phys. Rev. A 15 (1992) 7991. PJ F. Schuller. 0. Gorceix and M. Ducloy. Phys. Rev. A 47 (19931519. 141 M. Ducloy, Optics Comm. 99 (19931 336. 151 O.A. Rabi. A. Amy-Klein. S. Saltiel and M. Ducloy. Europhys. Lett. 25 (19941 579: A. Amy-Klein. S. Saltiel. O.A. Rabi and M. Ducloy. Phys. Rev. A 52 (19951 3101. if4V.G. Bordo. Sov. Phys. JETP 71 (19921 137. [71 V.G. Bordo. Surf. Sci. 269/270 (19921 169. 181V.G. Bordo. Optics Comm. 101 (19931 37. 191 V.G. Bordo. Optics Comm. 1 I I (19911 61. [lOI V.G. Bordo. Proc. SPIE 2547 (19951427. 1111 V.G. Bordo, Optics Comm. 127 (1996) 67.
V.G. Bordo et al. / Optics Communications
[12] M.F.H.
Schuurmans. J. Phys. (Paris) 37 (1976) 469, and references therein. [13] S. Le Boiteux, P. Simoneau, D. Bloch and M. Ducloy. J. Phys. B 20 (1987) L149. [14] P. Boissel and F. Kerherve, Optics Comm. 37 (1981) 397. [I51 P. Simoneau, S. Le Boiteux, C.B. de Araujo, D. Bloch, J.R.R. Leite and M. Ducloy, Optics Comm. 59 (1986) 103.
137 (1997) 249-253
253
[16] A.L.J. Burgmans, M.F.H. Schuurmans and B. BSlger, Phys. Rev. A 16 (1977) 2002. [17] F. Balzer. M. Hartmann, M. Renger and H.-G. Rubahn, Z. Phys. D 28 (1993) 321. [18] C. Henkel, Diplom Thesis. University of Gijttingen (1996). [19] F.O. Goodman and H.Y. Wachman. Dynamics of Gas-Surface Scattering (Academic Press, New York, 1976) p, 28.
OPTICS COMMUNICATIONS Optics Communications 137 (1997) 249-253
ELSEVIER
Evanescent wave fluorescence spectra of Na atoms V.G. Bordo ‘, C. Henkel, A. Lindinger, H.-G.Rubahn
*
Received 25 April 1996: revised 20 November 1996; accepted 30 December 1996
Abstract Laser induced fluorescence of Na atoms excited inside the evanescent wave of a glass prism is spectrally resolved for the first time, showing a pronounced dependence of linewidth on the surface temperature in the temperature range of 293 K to 435 K. We are able to reproduce the measured fluorescence lines quantitatively by use of the perturbative density matrix theory. The good agreement between theory and experiment emphasizes the fact that the response of the gas to the evanescent field arises mainly from the atoms desorbing from the surface and is of nonlocal nature. PACS: 32.50. + d; 32.70.J~; 42.50.-p
1. Introduction Resonant optics at a gas-solid interface has attracted growing interest during the last years [l-l I]. The excitation of gas atoms in the close vicinity of a surface results in a transient polarization behaviour of the atoms just having undergone a collision with the surface that leads to a nonlocal connection between the electric field and the induced polarization in the gas [ 121. Experimental evidence for such a behaviour was demonstrated via selective reflection spectroscopy from a glass-vapor interface, both in near-normal incidence [ 12.131 and total internal reflection geometries [ 14,15]. It was observed as a sub-Doppler structure in the reflection coefficient. The fluorescence spectrum excited by spectrally scanning a normally incident laser beam has also been studied [ 161.Another way to investigate these effects is to spectrally resolve the fluorescence, which has been excited by evanescent wave (EW) laser light. This approach facilitates direct comparison with theoretical predictions, which in turn show that the relevant spectra will display features connected with the atom-surface interaction [6-g]. We thus use the latter method in the present paper for the first time to demon-
* Corresponding author. E-mail: rubahnemsfdl .dnet.gwdg.de. ’ Permanent address: Institute of General Physics. Russian Academy of Sciences. 117942 Moscow. Russian Federation.
strate evidence for a nonlocal optical response of Na vapor atoms, which are resonantly excited by an evanescent wave at a glass prism surface. The measured fluorescence line shapes show good agreement with the theoretically predicted spectra and are described by the surface temperature. This means that the fluorescence signal arises mainly from the atoms thermally equilibrated at the surface and then desorbed into the gas phase. To our knowledge this is the first experiment allowing to discriminate the contribution of desorbing atoms in one-photon excitation, while in selective reflection this is possible only in three-level cascade scheme [5].
2. Theory The theory of resonance fluorescence from a gas of two-level atoms excited by an EW at a gas-solid interface was developed in a number of papers that deal with different regimes of the atom-EW interaction [6-g]. In order to compare the present experimental results with the theory two remarks are necessary. First. for a laser power of 400 mW the Na 3S,,, + 3P,,, transition is strongly saturated. Nevertheless perturbation theory with respect to the EW amplitude can still be applied for a quantitative description of the fluorescence line shape as long as the Rabi frequency for the relevant transition is considerably
0030.4018/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. P/I s0030-4018(97)00010-2
150
V.G. Bo& et trl./ Optics Cnrltrlllrrli(.L1tiOtl.~ 137 ( lYY7J24Y-253
smaller compared to the Doppler width. In this case. which is fulfilled for the present experiment. one can use the results obtained for a weak EW field [6,7] keeping in mind that this treatment does not give valid line irzrensitks. The second remark concerns the fact that the 3S,,2 + 3P,,, transition cannot be considered as a two-level system because of the hyperfine splitting of the ground state. The hyperfine splitting of the excited state is not resolved. However. as the laser is tuned in resonance with one of the 3s ,,? sublevels (F = I?), the radiation intensity arising from optical pumping to the other sublevel (F = I) is significantly less than the intensity of the stimulated emission at the resonant transition and can thus be neglected. The fluorescence spectrum might be determined by the Fourier transform of the atomic dipole moment two-time correlation function. The latter one is calculated here within a density matrix formalism. The master equation for the atomic density matrix has the form of optical Bloch equations containing the derivatives relative to the coordinate along the normal to the interface. To solve this set of differential equations one must specify separate boundary conditions for atoms moving towards and away from the surface. The solution for the atoms departing from the surface is determined by the sticking probability of a gas atom at the surface, S. We assume S = I. which is a reasonable assumption for Na atoms which hit a dielectric surface at room temperature [ 171. The resulting fluorescence spectrum can be written in the form [6]
g(W’)=g,(w’)+g:(w’)+g:,~w’).
(1)
T(L’)=y~KL’,+i(d+cl’.,).
- K 1‘.
’
(KL’;)2+(Or--W+p.ZJ)2
/+T 7’ where j--
6
(2)
(3)
r) ’
and
I X
(8)
L’7+
3. Comparison
KI‘; + i( w’ - w +p
(6)
P(s) = I’(L’,‘(‘,._ (..I, d = w,, - w, y = T- ’ with T the transverse relaxation ;ime of the atomic transition, K = fi- ’ with 6 the exponential decay length of the EW and p = q - k. We have assumed that the EW propagates along the .V axis and has wave vector q, and that the observed fluorescence light has wave vector k and frequency w’. The fluorescence spectral density has three contributions. Firstly. a contribution from atoms which approach the surface. g;. Secondly, a contribution from atoms which desorb from the surface. a,‘. Both contributions are centered near the EW frequency, w. Finally, there is a contribution from desorbing atoms, which give rise to fluorescence light, which is centered near the atomic transition frequency. w,,. The latter term is caused by the trmsiertt behaviour of the desorbing atoms and is thus correlated with the nonlocal response of the gas atoms near the surface [I’?]. The relative weight of this contribution can be estimated by the ratio
is the mean time-of-flight across the EW field. Under our experimental conditions, 6 = 245 nm. ~7 = 500 m/s, T = 33 ns, we find R = 67. Therefore the third, nonlocal term dominates the experimentally determined fluorescence spectrum.
where
X
is a Maxwellian distribution function with 1‘7_ the most probable velocity of atoms approaching to (departing from) the surface.
(4)
y+i(w’-wO-k.Z’)
Here, we have introduced the following (~‘,,L.,.,v_) is the atomic velocity,
notations:
z’ =
with experiment
The experimental set up (Fig. I) consists of a glass prism. which is mounted inside a vacuum chamber ( pc,I 10m7 mbar) on a heatable manipulator. A flux of Na atoms from a resistively heated oven CT,,,, = 428 K) reaches the prism surface under an angle of 45” with respect to the surface normal. The light from an Art laser pumped single mode ring dye laser (CR 699-21) irradiates the prism via a Brewster angle window at an angle larger than the angle of total internal reflection. It leaves the vacuum apparatus through another Brewster angle window. The most important source of stray light stems from light which is scattered from defects on the prism surface. This background light intensity has been measured to be significantly less than 10-j of the incoming light intensity [ 181. The dye laser is set at a fixed frequency of 16973.33(l) cm-‘. where the error results solely from the spectral
V.G. Bordo rt al. / Optics Conmunioatioru
evanescent
\/
137
f 1997)
249-253
wave
photomultiplier
Fig. 1. Scheme of the experiment: 3S-3P resonance fluorescence of Na atoms (h us,,, ) is excited (hr;,,) within the evanescent wave of a glass prism and is spectrally resolved using an etalon with a FSR of 7.5 GHz and a finesse of 100. The UHV machine is decoupled from the detector using an optical fiber.
resolution of the wavemeter. The FWHM of the laser is 6.7 X 1O-5 cm-’ with a drift of far less than 3 X IO-” cm-’ during an etalon scan. The laser light is used to excite Na atoms in the EW above the surface via the 3S, ,?( F = 2) + 3P,,,(F) transition. The resulting fluorescence light is observed via a collection lens at an angle of 45” with respect to the surface normal and is coupled into a multimode optical fiber. The light ouput of the fiber is directed into a Fabry-Perot etalon with a FSR of 7.5 GHz, which is scanned via a mirror mounted on a piezoelectric crystal. The transmission of the etalon is recorded by a photomultiplier and photon counting electronics. Accurate alignment of the etalon results both in a high finesse (F = 100 in the present case) and maximum signal-to-noise ratio. This alignment is achieved by a decoupling vacuum apparatus and detection optics via an optical fiber and prealigning the etalon using single mode laser light. In Fig. 2 a typical spectrum obtained from fluorescing atoms in the evanescent wave above the prism is pre-
-4 -3 -2 -1 0
1 2 3 4 5 6
Detuning [GHz] Fig. 2. Spectrally resolved fluorescence of Na atoms (3S,,? (F=?) + 3P,,> (F)). excited with a single mode dye laser at 16973.33( 1) cm _’ in the evanescent wave of a glass prism CT = 430 K. P,_ = 400 mW).
I -3000
I
I
I
-2000
-1000
0
I
Detuning (MHz) Fig. 3. Same as Fig. 2. but for surface temperatures of 293 K (a) and 435 K (b). The solid lines result from the theoretical approach as described in the text.
sented. The surface temperature was 430 K. The sharp line results from light which is scattered at defects on the surface of the prism. The broad line on the left-hand side of this peak is the fluorescence spectrum from desorbing Na atoms. As shown in Fig. 3. the width of this latter line depends on the surface temperature. Note that in contrast to usual gas phase spectra the fluorescence spectrum observed via evanescent wave excitation consists of only ~fre tluorescence line. To ensure that we are observing mainly atoms excited inside the evanescent wave and not in the gas phase the exciting laser beam was aligned to propagate parallel to the prism and the fluorescence light was spectrally resolved. Changing the temperature of the prism, we did not observe any change in the fluorescence line shapes, in contrast to the spectra observed in the evanescent wave configuration. In the same instance changing the temperature of the Na oven and thus the flux of incoming Na atoms did not change the spectra, which were recorded in the EW configuration. The experimental spectra are compared in Figs. 3a and 3b withfluorescence spectra calculated from Eq. (4) for different prism temperatures. Theoretical and experimental fluorescence intensities have been normalized at the position of the fluorescence peak. The theoretical value of the transition frequency is 16973.34 cm-’ in both cases, which is within error bars with the experimentally used
V.G. Bordo er al. / Optics Communicntions
252
I
I
280
I
320
I
360
I
400
440
Surface temperature [K] Fig. 4. Measured width (FWHM) of the spectrally resolved fluorescence of Na atoms in the evanescent wave as a function of temperature of the prism surface. The solid line results from numerical calculations using Eq. (4) of the text.
value. Obviously the experimentally determined line shapes are well described by the theoretical profiles obtained for nonlocal contribution for both low and high prism temperatures. In Fig. 4 the measured half widths of the fluorescence lines are plotted as a function of prism surface temperature. Within error bars both the absolute magnitude of half width as well as the increase with increasing temperature are well represented by the theoretical approach using Eq. (4). It should be stressed that we have essentially nonequilibriumconditions at the surface. The atomic flux incident on the surface from the gas phase is dictated by the vapor pressure inside the Na oven (= IO-’ Torrl. resulting in a mean free path A of the order of a few cm. The linear dimensions of the flux of Na atoms are much smaller than A. This means that the atoms collide with the surface much more frequently than with each other. Under such (“collision-free”) conditions thermodynamical equilibrium cannot be established for the atoms near the surface [19]. Therefore, there are two atomic fluxes in the vicinity of the surface: (i) incident on the surface and determined by the oven temperature, and (ii) desorbing from the surface and determined by the surface temperature. Thus the atomic flux desorbing from the surface gives the only contribution to the fluorescence intensity which can be associated with the prism temperature.
4. Conclusions In this paper, we have investigated the fluorescence spectrum of Na atoms excited by an evanescent wave at a glass prism surface. The exponential decay length of the EW was significantly smaller than the flight path of the Na atoms during the relaxation time. This condition leads to rronlocality of the optical response of the gas atoms near the surface. As a direct consequence the fluorescence
137
ClYY7J 249-253
spectrum contains a component arising from atoms that have been desorbed from the surface. This component is centered near the atomic transition frequency. In our case, this line dominates in magnitude. Its appearance is a novel manifestation of the nonlocal optical response of the gas in the close vicinity of a surface while in selective reflection nonlocality leads to a sub-Doppler structure in the reflection coefficient. Experimentally, this prediction has been checked by spectrally resolving the fluorescence of Na atoms excited within the EW of the prism. Using thermodynamically nonequilibrium conditions at the surface (different oven and prism temperatures) and “collision-free” atomic flux, we have demonstrated that the fluorescence line shape is determined by the surface temperature and does not depend on the temperature of the incoming flux. Thus the fluorescence indeed arises mainly from the atoms leaving the surface. This method opens new opportunities to study desorption processes from the surface into the gas atmosphere. In particular, the integral intensity of the fluorescence line is determined by the atomic tlux desorbing from the surface. Thus the temperature dependence of this signal will provide information concerning the desorption rate and hence, the heat of adsorption. Measurements of such kind are in progress.
Acknowledgements We are grateful to the Deutsche Forschungsgemeinschaft for financial support. which enabled the joint work presented here. We also thank J.P. Toennies for encouragement and generous support and the European Community (network “Laser controlled dynamics of molecular processes and applications”) for partial financial support. One of us (V.G.B.) is grateful to the Max-Planck-Institut fur Stromungsforschung for hospitality.
References
[II M. Chevrollier.
M. Fichet. M. Oria, G. Rahmat. D. Bloch and M. Ducloy. J. Phys. II (Paris) 2 (19921 63 I. El A. Weis. V.A. Sautenkov and T.W. Hansch. Phys. Rev. A 15 (1992) 7991. PJ F. Schuller. 0. Gorceix and M. Ducloy. Phys. Rev. A 47 (19931519. 141 M. Ducloy, Optics Comm. 99 (19931 336. 151 O.A. Rabi. A. Amy-Klein. S. Saltiel and M. Ducloy. Europhys. Lett. 25 (19941 579: A. Amy-Klein. S. Saltiel. O.A. Rabi and M. Ducloy. Phys. Rev. A 52 (19951 3101. if4V.G. Bordo. Sov. Phys. JETP 71 (19921 137. [71 V.G. Bordo. Surf. Sci. 269/270 (19921 169. 181V.G. Bordo. Optics Comm. 101 (19931 37. 191 V.G. Bordo. Optics Comm. 1 I I (19911 61. [lOI V.G. Bordo. Proc. SPIE 2547 (19951427. 1111 V.G. Bordo, Optics Comm. 127 (1996) 67.
V.G. Bordo et al. / Optics Communications
[12] M.F.H.
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