fluorescence microscopy and spectroscopy of an isolated micro-droplet

ELSEVIER Materials Science and Engineering B48 (1997) 139-146

Fluorescence microscopy and spectroscopy of an isolated micro-droplet

Stephen Arnold *, Stephen Holler, Noel L. Goddard Microparticle Photophysics Lcrh (MP ‘L), Polytechnic Uniwrsity, Brooklyn, NY I l-701. USA

Abstract

Fluorescence microscopy and spectroscopy experiments are carried out on individual microdroplets (p-drops) isolated within an electrodynamic levitator-trap (Paul type) at atmospheric pressure. Imaging experiments on spatially homogeneous donor and acceptor dyes within particles of radius a - 10 pm reveal a ‘global’ transfer mechanism with a yield exceeding conventional dipole-dipole transfer (Fiirster transfer) by - 1000 x between molecules at a particular radial distance r > 0.93 (S. Arnold. S. Holler, S.D. Druger, J. Chem. Phys. 104 (1996) 7741). but is consistent with the quantum theory of enhanced energy transfer through morphology dependent resonances (MDRs) of the sphere (P.T. Leung, K. Young, J. Chem. Phys. 89 (1988) 2894). To gain a better understanding for the primary step in this process, we investigate fluorescence from a particle with a dilute layer of surface active molecules. These surfactant molecules have a fixed orientation relative to the surface as revealed both qualitatively from a heuristic interpretation of polarization analyzed images (S. Arnold, L.M. Folan. Proc. SPIE 1862 (1993) 218). and quantitatively through the preferential cavity enhanced selection of various polarized modes in emission spectroscopy. The spectra are adequately described through semi-classical theory (S.D. Druger, S. Arnold, L.M. Folan, J. Chem. Phys. 87 (1987) 2649). This theory has also been applied to the description of reported .anomalous’ fluorescence lifetime measurements (M.D. Barnes, C.-Y. Kung, W.B. Whitten, J.M. Ramsey, S. Arnold, S. Holler. Phys. Rev. Lett. 76 (1996) 3931) from a dilute surface layer. Although sum rules based on density of states of high Q whispering gallery modes fail to describe the data at particle sizes for which the homogeneous linewidth is larger than the free spectral range (M.D. Barnes, C.-Y. Kung, W.B. Whitten, J.M. Ramsey, S. Arnold, S. Holler, Phys. Rev. Lett. 76 (1996) 3931). semi-classical theory, including all modes, is in considerably better agreement with the data. 0 1997 Elsevier Science S.A.

Keywords: Microparticle; Cavity quantum electrodynamics; Fluorescence: Levitation

1. Introduction

Photophysical experiments on spherical particles have received a great deal of attention in recent years [6]. Interest at the MP’L in fluorescence from molecules in aerosol p-drops began in 1984. It was motivated in part by the discovery of Chang and co-workers [7] that sharp peaks, associated with Mie resonances (a.k.a. morphology dependent resonances, MDRs) appear in the fluorescence emission of dyed hydrosol polymer beads. We asked, ‘could such an effect alter chemical physics in an aerosol particle?’ We started by examining intermolecular energy transfer (ET) and almost immedi- ately began to see transfer well beyond ( - 100 x ) that expected from the most efficient traditional mechanism [8] (i.e. the near field dipole-dipole (d-d) interaction

* Corresponding author. e-mail: [email protected]

0921.5107/97/$17.00 Q 1997 Elsevier Science S.A. All rights reserved PII so921 -5 107(97)00094-9

[9]). The effects observed are most easily described by using ideas associated with cavity quantum electrodynamics (CQED) [1,2]. This paper starts by discussing our approach to fluorescence measurement. Following this we discuss some of the ET results within homogeneous p-drops and extend the work on fluorescence to oriented molecules at the droplet surface.

2. Experimental approach

Our studies are currently performed in the aerosol particle microscope-spectrometer (APMS) [lo] shown in Fig. 1.

A charged glycerol p-drop is held in a temperature controlled electrodynamic levitator-trap (ELT) in air [II]. The trap is of a Paul type [12] consisting of three electrodes, top and bottom hyperboloids of revolution

140 S. Arnold er al. :Murrriuls Science and Engineering B48 (1997) 139 146

TIT CCD

Microscope

CCD Detector + “dc’*

From r\

f Controller >

- -.” solvent tieservolr

* v Picopipette “p4

Hg Arc

365 nm

l/4 h Plate 408 nm

Fig. I, Aerosol particle microscope-spectrometer.

(interelectrode spacing, 9 mm), and a central torus having a hyperbolic cross section. To eliminate stray fields at the center small pin electrodes have been added to the central torus and electrified appropriately [ 13,141. With this modification a particle with a = 7.5 urn can be trapped at room temperature with a variance of gyr =0.35 urn [13-151. The images which we will present in connection with ET and surface molecular fluorescence have a spatial resolution limited only by the optical resolution of the microscope and the pixel resolution of the CCD camera; effective resolution = 0.9 urn.

Charged particles are injected after withdrawing the microscope barrel vertically and positioning a picopipette [l l] just above the top hyperboloid. The charged u-drop is prepared by: (1) drawing a glycerol/ water(methanol)/dye solution through a micron sized orifice of the picopipette, (2) squeezing a few picoliters from the orifice by momentarily electrifying piezoelec-

tric strips attached to the pipette body, and (3) charging the droplet inductively as it is emitted. Once trapped, in dry air, most of the water (methanol) evaporates leav- ing a single dyed glycerol droplet. The microscope barrel is then lowered in order to focus an image of the particle on the CCD, and on the entrance slit of our spectrometer.

The particle is surrounded by a variety of excitation sources (Fig. 1). A Hg arc (365 nm) and a He-Cd laser (442 nm) are set up for side illumination (‘side-lit’). An Ar+ laser (488 nm) back illuminates the particle with circularly polarized radiation (‘back-lit’).

3. Intermolecular energy transfer (ET)

In ET a donor molecule D is excited by light (D + hv -+ D*) and subsequently transfers its energy to an acceptor (D * + A -+ A * + D) at a separation r by some

S. Arnold et al. /‘Marerials Science and Engineering B48 (1997) 139-146 141

mechanism. Traditional intermolecular energy transfer in bulk solutions was discovered by Perrin [16] 70 years ago and later described by Fiirster [9]. Fiirster transfer occurs through a dipole-dipole (d-d) interaction in the near field. Its rate is proportional to the overlap between the donor emission spectrum and the acceptor absorption spectrum. The transfer efficiency can exceed 50% for Y - 50 A and varies - l/r’. Radiative transfer, which clearly will have a larger range, is considered ‘trivial’ because of the small solid angle subtended by a molecular absorber as seen from the donor; e.g. for a radiating donor molecule at 10 pm from an acceptor having an absorption cross section of 1 A2, the solid angle subtended by the acceptor is - lo- lo steradians!

ET in an isolated p-drop changes radically. Fig. 2(a) shows a low resolution spectrum of a soluble donoriac- ceptor dye system in a glycerol p-drop with a - 10 pm [D/A system, 5 x 10 - ’ M 9-aminoacridine (9AA)/ 10 ’ M Rhodamine 6G (R6G)] at room temperature. The particle is side-lit at 365 nm using a Hg arc (intensity < 50 mW cm - 2), and the spectrum is accumulated over 1 min. The surprising thing about Fig. 2(a). is the peak at the right in the particle spectrum. This emission maximum of R6G, should not have been apparent. At a concentration of 10 - 6 M the average separation

- (a) 1. 5~10-~M 9AA Bulk 11. 5~10-~/1O~~M 9AA/R6G Particle

340 406 472 536 604 670

Wavelength (nm)

545 555 565 575 Wavelength (nm)

Fig. 2. (a) Comparison of bulk donor, and the particle mixture emission spectra for the 9AAiR6G system. (b) A portion of the fluorescence spectrum of 9AA/R6G system in a glycerol microdroplet along with the molar extinction spectrum.

between a donor and its nearest neighbor acceptor is nearly 1200 A. By summing all neighboring d-d inter- actions we would predict that the R6G contribution to the spectrum would only be - O.l%, whereas the energy contributed by R6G to the entire spectrum is z 15%. Another soluble D,!A system, Coumarine 1 (Cl)/R6G was tried, and it gave similar results. In addition, for both systems the ratio of acceptor to donor fluorescence R showed a similar dependence on acceptor concentration y, over the range 10 ’ M < pa < 3 x 10 - 5. Below 10 ’ M, R increased in propor- tion to pa, and above 5 x 10 ’ M it began to saturate to a value - 0.1 [17].

To record the spectrum of a sphere rather than the average of many spectra from an evaporating drop, the temperature in the levitator was dropped by 20°C and the resolution was increased (0.08 nm). The R6G portion of the spectrum in the 9AA/R6G system (Fig. 2(b)) revealed a possible mechanism for the large amount of transfer. In the tail of the spectrum as well as at shorter wavelengths (emission principally due to 9AA, not shown) resonances appear (Fig. 2(b)) which can be identified from their frequencies to be at the same energies as Mie resonances.

Mie resonances (a.k.a. MDRs), which are given their name due to their appearance in elastic light scattering, correspond to natural electromagnetic oscillations of a dielectric sphere. They may be thought of in terms of rdyS within the interior which orbit in closed polygons while being trapped by ‘nearly’ total internal reflection. MDRs are optically mesoscopic in our size range and are usually described by their standing wave description: P;’ where P is the mode polarization (transverse magnetic, TM, or transverse electric, TE), I is the angular momentum quantum number and v is the radial quantum number (the number of peaks in intensity between the center and the surface). The azimuthal quantum number m has been suppressed in our nota- tion since all values of M for a given I (i.e. 21f 1 integers in the interval - I < r?? < I) are degenerate for a sphere. MDRs are particularly significant in connection with the enhancement seen in Fig. 2(a) since their appearance indicates that energy is being coupled into the particle interior (by both excited donors and acceptors). Furthermore, although the photon lifetime within a mode is sensitive to both v and 1, an unloaded resonance in a glycerol particle 10 pm in radius can have a Q approaching 5 x 10”; the photon has an effective path length L k 30 cm! The broadening of these high Q resonances with increased R6G absorption in both the R6G (Fig. 2(b)) and 9AA portions of the spectrum (not shown), points to an alternate transfer mechanism to near field d&d coupling. The donor can transfer energy to an acceptor with the MDRs as a conduit.

142 S. Arnold et 01. fMuteriulv Scienw and Engineering B48 (1997) 139 146

Fig. 3. Images taken of a C334:SRlOl microdroplet in donor (left) and accepter (right) fluorescence

Putting all of the responsibility for the enhanced energy transfer in Fig. 2 on MDRs may seem mis- guided when one considers that MDRs only appear to make up a small portion of the spectrum in Fig. 2(b). Another possible hypothesis is that D-A clusters form at the charged surface of the particle, where they transfer energy by the Fiirster mechanism. The loss of transfer with reduced concentration could simply be due to cluster melting.

With the microscope in Fig. 1 these two hypotheses are easily separated. Relative to communication through ‘global’ electromagnetic modes (MDR) which extend over the entire diameter of the droplet, Forster transfer is local (10 pm vs. 50 A). Thus, images taken separately in donor and acceptor fluorescence should look identical for the Forster mechanism, whereas a remarkable difference should appear based on the a mechanism associated with ‘molecular telegraphy’ through MDRs.

Fig. 3 shows images of a u-drop (~12 9.5 pm) in donor and acceptor fluorescence. This D/A system is composed of 10 ’ M coumarine 334 (C334)/5 x 10 ” M sulforhodamine 10 1 (SR 101). The droplet was side- lit with a He-Cd laser (442 nm Fig. 1). The image in Fig. 3(left) was acquired over 30 s by filtering the fluorescence with glass filters to isolate the donor emission (DF filter). Another image was recorded using a set of filters which preferentially transmit acceptor fluorescence. This image contained a small percentage of the donor luminescence, however this ‘ghost’ donor image was eliminated using an electronic image subtrac-

tion [l]. The resulting acceptor image in Fig. 3(right) (‘AF’ Filter) leaves little doubt as to the origin of the energy transfer.

What we are seeing in the acceptor image is the stimulation of high e MDRs by excited donors within an ‘active region’ beyond r - 0.93 a and having an intensity weighted volume fraction of lo-15% [l]. As a consequence the front to back asymmetry which is apparent in the donor image is lost from the acceptor image. In addition the acceptor image clearly shows that the best transfer occurs between diametrically op- posed donors and acceptors. Photometrically, acceptor photons comprise 2 7.5% of all the emission from the sphere. Since all visible transfer takes place within a small volume fraction of the particle, the average transfer efficiency between donors and acceptors in this volume is considerably larger than the overall efficiency of 7.5%. A more reasonable number would be 2 50%; - 1000 x that expected from Fiirster transfer.

Since our original work a number of theories, both semiclassical [4,18] and quantum mechanical [2,19], have been written to describe ‘MDR enhanced energy transfer’. Three are based on a particle-particle interaction formalism [4,18,19], while the other utilizes con- cepts associated with CQED [2]. We will compare the latter with our experiments.

The quantum mechanical model is based on weak coupling and irreversibility [2]. Although MDR enhanced energy transfer is treated by 2nd order pertur- bation theory in the presence of an interaction involving field operators for MDRs and dipole opera-

S. Arnold et al. /Materials Science and Engineering B48 (1997) 139- 146 143

tors for both the donor and acceptor [i.e. c= PD. ,!?.(c o) + PA. E(Y~)], the physics which evolves can be pictured simply as a two step process. In the first step the excited donor succumbs to quantum fluctua- tions in the local field associated with an MDR and emits a photon into the MDR. The photon resides within the MDR until it leaks out into the rest of the universe with rate ~(11, I) = w/Q(r, 1) or is absorbed by an acceptor molecule with rate )le = ~,(o)p, c/n, where 0, is the molecular absorption cross section.

The competition for donor emitted photons between absorption on the one hand and leakage on the other provides an explanation for the observed concentration dependence. The probability for absorbing an emitted photon P = y,/[y, + II(V, 1)]. Saturation (i.e. P z 1) occurs for ‘absorption dominated modes’ with y, >> y( v, I), whereas when the opposite is true the probability will be proportional to y, or pa. From the concentration dependence measured in our experiments and the average molecular absorption cross section in the region of overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, (g,(tc))), we find that the longest lived mode contributing to transfer in our glycerol particles (a - 10 urn) has a Q - 4 x 106. Interestingly this Q is a factor of 50 below the largest Q calculated from Mie theory for our particle’s refractive index, 12 = 1.47, and nominal size. Cur- rently it is thought that dynamic surface roughness is responsible for much of this disparity.

Although saturation is controlled by competition between absorption and leakage, it does not define the efficiency with which the MDRs are stimulated. A description may be devised based on the Fermi Golden Rule as applied to emission in a cavity. The rate of emission within the cavity at position p and frequency tc) is

where M is the electronic matrix element and p(o, c) is the density of final photon states. By describing donor emission with an appropriately normalized spectral function @o(o), seen in Fig. 4, the emission rate aver- aged over the cavity volume in comparison to the rate in bulk is [20]

1 VC -

07 _ vc 0 dV 0 -J s x @d~)~(wr) do

TO

s %

(2) %,(w)po(w) du

0

Eq. (2) describes the radiative decay rate into all modes. For a single narrow MDR which resonates at CL), with a line width 60, and a density of states ps((ti, L), we simplify the integration by recognizing that C+,(w) varies slowly enough to be replaced by @i,(w,), and (po) = (w’)n3/(n2c3). In addition, the integral of the

density of states, over both frequency and volume, is approximately the mode degeneracy; Sum Rule 1:

$ I dV ps(w, y) dw = D, = (21+ l),

VC h s

[21]. Thus the rate of radiative decay into an MDR as compared to radiative decay rate in bulk, R,, is

R s

= 0-s) -= %I(%)4 _ 7r2c3 ,(2/f l),@,,(ws)

r. V&o> n’Vc <w’> (3)

The ratio R, has particular significance for our prob- lem when one realizes that the average decay rate for a molecule in the particle is the essentially the same as that in bulk as long as the emission spectrum is broader than the separation between resonance, Aw. To prove this use Sum Rule 2:

l <P(W r>>v dw =

s P,(O) dw.

AU AW

[21]. Thus, for an absorption dominated mode S, Eq. (3) represents the fraction of donor emission which is transferred by the mode. The overall efficiency R is found by summing Eq. (3) over all absorption dominated modes in the overlap region. Leung and Young [2] have performed just such a sum with the result

R ~ 7-12c3(21 + l)S,, f n3Vc(02) (4)

where S,, is the average spectral density (angular) of absorption dominated modes, and f is the ratio of the width of the overlap spectrum to the width of the emission spectrum. There have been extensive calculations of the number of modes of various widths in p-spheres [22]. For a particle of glycerol 9.5 urn in radius ( V = 3.6 x 10 ~ 9 cm’), with n = 1.47, the number of resonances which are long enough lived to be absorption dominated (Q > S x 10’) at our acceptor con-

Frequency, (11

Fig. 4. Relevant spectra for describing MDR enhanced energy transfer in a microparticle.

144 S. Arnold rt al. :Matrriab Sciencr and Engineering B48 (1997) 139- 146

A Polarizer Transmission

Filter p&i&--- axis

of polarizer .---__--_- 1

Fig. 5. Images of ODRB in a glycerol p-droplet as seen without (left) and with (right) a polarizer

centration and limited in Q to the upper limit measured in glycerol (Q,,, < 10’) have an average spectral density of l/33 mode per cm ~ ’ and an average degeneracy of (21f 1) zz 335. The ratio f for our C334iSRlOl system is z 1, By taking (w ) as the frequency at the center of the overlap region, 2rc x 5.25 x lOI Hz, we find R = 0.11, which is in good agreement with our overall measured efficiency of 778%.

It should be pointed out that we assumed in our calculations that excited donors and unexcited acceptors are spatially uniform just following light absorption. If we had ‘tailored’ our particle so that all dye molecules were placed at the center of the active region, within u - /i from the surface, we would expect considerably more energy transfer, R 2 50% [4].

A surprising aspect of enhanced ET in u-drops is that theory predicts, for our experimental conditions, that the effect should be accompanied by only a small change in excited state decay rate. The reason is a competition between the homogeneous linewidth I,, of the donor, and MDR free spectral range Aw. The excited state decay rate is expected to be enhanced by as much as A~,ir,, for Aw > I,,, however when Ace < I-,,, essentially no enhancement is expected [21]. Our experiments are carried out in a size region in which the free spectral range Aw z c/(na) is nearly the same as the homogeneous linewidth [23]. As it turns out this justifies the use of the broader CD,(w) in the analysis of our observed ET effects. It also suggests that energy transfer experiments on smaller particles should lead to larger ET enhancements.

4. Surface molecular fluorescence

So far our dye molecules have been treated as if they were atoms. Even though molecules have body fixed emission moments we have assumed that mode accessi- bility is independent of the direction of the molecular transition moment. In an approximate sense, the ran- dom molecular orientations of the dye molecules in the liquid help to justify our approach. We also have not accounted for spatial inhomogeneities in the excitation. A simpler and more definitive situation for testing theory would be to place oriented molecules in a droplet at a known radius. The choices are simple, the only place where the symmetry is broken sufficiently for molecules to orient is at the surface, a region where surfactant molecules find a natural home [3].

Two surfactant species were chosen, DiI(3) and oc- tadecyl rhodamine B (ODRB) [both from Molecular Probes], at concentrations consistent with a dilute surface layer. Each was dissolved in glycerol by the addition of methanol. After generation of a charged droplet with a picopipette, the levitated particle loses the major- ity of its methanol component through evaporation, and as a result the surfactant species are driven to the surface.

Fig. S(left) shows a typical image of an ODRB doped particle back-lit with a circularly polarized beam from an Ar + laser at 488 nm (Fig. 1). A rim of fluorescence is seen around the edge as well as a spot due to droplet focusing at the center. Fluorescence emitted from the rim is distinctly polarized as shown in Fig. S(right). The surface tangent of the rim perpendicular to the polar-

S. Arnold e! al. /Materials Science and Engineering B48 (1997) 139- 146 145

izer axis is nulled, indicating that the molecules have emission moments tangent to the surface. Both DiI and ODRB had similar images.

Now that we have an extremely well defined system, there are a number of questions which can be asked. First and foremost, can one see the principle signature of CQED? This signature, as pointed out by Yokoyama and Brorson [20], is the dependence of the excited state decay rate on size, a. Measurements at the Oak Ridge National Laboratory (ORNL) by Barnes et al. [5] using picosecond lifetime techniques are shown in Fig. 6. There is a pronounced increase in decay rate as the size is reduced. However, its form does not fit expectations.

The expectations based on [20] are that the enhancement over the bulk rate of decay could be as large as

m-0 -z A&l/I-, - l/a. By pinning this hyperbolic size dependence to one point (at a z 2 pm) calculated semi- classically, using a 100 cm - ’ wide homogeneous line positioned for optimum effect, the solid curve in Fig. 6 arises [5]. As one can see, the inverse size dependence does not work out. In addition, there is a 30% depres- sion in the rate of decay, which is unexpected; above a diameter of 20 pm the homogeneous line width I-, is larger than the free spectral range Aw and our expecta- tion based on sum rules are that I-/r0 should approach 1 asymptotically.

The ideas which have guided us thus far were based on mode counting with no regard to molecular orientation, and sum rules which are based on volume aver- ages. Each of these will have to be modified when thinking about the orientation of molecules at the surface of a p-drop. Until then we have taken a semi- classical approach in an attempt to describe the data in Fig. 6.

We represent the molecular excited state of ODRB as an oscillating dipole polarized parallel to the interface.

2.5 1 ,

0.5 " ' "I' )"I"" I " 3 'I""

0 5 10 15 20 25

drop diameter (2a, pm)

Fig. 6. The rate of decay of ODRB in a dilute layer on a glycerol p-drop.

Since there is very little absorption at the emitting wavelengths we calculate the rate of decay relative to the bulk rate by allowing the dipole to be driven by its own self generated scattered field. The field generated at !- by a dipole at c’ is E(r) = G(c, I’, w)~ where G(r, r’, w) is the appropriate dyadic Greens function for a dipole within the sphere [24]. The relative rate of decay for this weakly damped system is

where ti is a unit vector in the direction of the dipole and and k = wjc. Druger et al. [4] have anticipated the incorporation of a molecular line shape function h(o), which has been chosen as a frequency normalized Lorentzian having a line width rh. On this basis

(6)

To test this approach we first calculated (T)/TO at the large size extremes in Fig. 6. We assumed a width for h(w) which is larger than the free spectral range between MDRs. Our results for u = 10, 11 and 12 pm are essentially equal, (r)!T(, 2 0.65. This result is in good agreement with experiment and shows the effect of incorporating all partial waves, resonant and nonres- onant. into the Green’s function. The disparity between this result and the CQED approach of mode counting and sum rules is likely to be due to leakage of states near the surface. In effect this establishes a new base- line. The dashed curve shown in Fig. 6 connects up our or calculations at 10, 11 and 12 pm with an optimal calculation at 2 pm using a l/u interpolation. Although this is in far better agreement with the data than the solid curve, its curvature is wanting. It appears that higher order dependences on 1 /cc may be important (e.g. an additional component which - (11~1)‘).

The value for (r)jr,, at 2 pm is very sensitive to the choice of homogeneous linewidth. For half or twice the value chosen it rises or falls by nearly a factor of 2, indicating that one can effectivey us the CQED effect to estimate the homogeneous linewidth at room temperature [23].

5. Future directions

We have merely scratched the surface in our investigation of phenomena associated with CQED and sol- vated organic chromaphores. It would be interesting to construct a monolayer at the surface of a sphere and investigate exciton transport in the presence of the CQED effect. Spectroscopy can be expected to define the orientation of transition moments [25], and low temperature experiments would allow for an more sensitive investigation of the effect of changing the homogeneous line width.

146 S. Arnold et al. / Materials Science and Engineering 848 (1997) 139 146

Acknowledgements

We thank L.M. Folan of the Polytechnic for his preliminary results in connection with fluorescence imaging, and N. Wotherspoon of the MP3L for devel- oping the picopipette. We are grateful for support from the National Science Foundation under grant ATM-X9- 05871.

References

[I] S. Arnold, S. Holler, S.D. Druger, J. Chem. Phys. 104 (1996) 1741.

[2] P.T. Leung, K. Young, J. Chem. Phys. 89 (1988) 2894. [3] S. Arnold, L.M. Folan, Proc. SPIE 1862 (1993) 218. [4] S.D. Druger. S. Arnold, L.M. Folan, J. Chem. Phys. 87 (1987)

2649. [S] M.D. Barnes, C.-Y. Kung, W.B. Whitten, J.M. Ramsey, S.

Arnold, S. Holler, Phys. Rev. Lett. 76 (1996) 3931. [6] See articles within Optical Processes in Microcavities, R.K.

Chang and A.J. Campillo (Eds.), World Scientific Publishing, 1996.

[7] R.E. Benner, P.W. Barber, J.F. Owen, R.K. Chang, Phys. Rev. Lett. 44 (1980) 475.

[8] L.M. Folan. S. Arnold. SD. Druger, Chem. Phys. Lett. 118 (1985) 322.

[9] Th. Forster, Ann. Physik 2 (1948) 55. [IO] S. Arnold, S. Holler, J.H. Li. A. Serpengtizel, W.F. Auffermann,

SC. Hill, Opt. Lett. 20 (1995) 773. [II] S. Arnold. L.M. Folan, Rev. Sci. Inst. 57 (1986) 2250. [12] R.F. Wuerker, H. Shelton, R.V. Langmuir, J. Appl. Phys. 30

(1959) 342. [13] S. Arnold, L.M. Folan, A. Korn, J. Appl. Phys. 74 (1993) 4291. [14] S. Arnold, J.H. Li, S. Holler, A. Korn, A.F. Izmailov, J. Appl.

Phys. 78 (1995) 3566. [15] A.F. Izmailov, S. Arnold, S. Holler, A.S. Myerson. Phys. Rev. E

52 (1995) 1325. [16] J. Perrin. CR. Acad. Sci. 184 (1927) 1097. [17] S. Arnold, L.M. Folan, Opt. Lett. 14 (1989) 387. [I81 A.C. Pineda, D. Ronis. Phys. Rev. E 52 (1995) 5178. [19] M.S. Kurdoglyan, Opt. Spectrosk. 75 (1993) 488. [20] H. Yokoyama, S.D. Brorson, J. Appl. Phys. 66 (1989) 4801. [21] SC. Ching, H.M. Lai. K. Young, J. Opt. Sot. Am. B 4 (1987)

1995. [22] SC. Hill, R.E. Benner, J. Opt. Sot. Am. B 3 (1986) 1509. [23] M.D. Barnes, W.B. Whitten. S. Arnold, J.M. Ramsey, J. Chem.

Phys. 97 (1992) 7842. [24] H. Chew, J. Chem. Phys. 87 (1987) 1355. [25] S. Arnold, S. Holler and N. Goddard (in preparation).

fluorescence microscopy and spectroscopy of an isolated micro-droplet

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