Absorption spectroscopy on single molecules in solidsLothar Kador, Tatiana Latychevskaia, Alois Renn, and Urs P. Wild

Citation: The Journal of Chemical Physics 111, 8755 (1999); doi: 10.1063/1.480222 View online: http://dx.doi.org/10.1063/1.480222 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/111/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Infrared laser spectroscopy of the CH 3 – HCN radical complex stabilized in helium nanodroplets J. Chem. Phys. 124, 104305 (2006); 10.1063/1.2170087 Role of rare sites in single molecule spectroscopy measurements of spectral diffusion J. Chem. Phys. 114, 10479 (2001); 10.1063/1.1373688 High resolution infrared absorption spectra of methane molecules isolated in solid parahydrogen matrices J. Chem. Phys. 111, 4191 (1999); 10.1063/1.479717 Absorption, excitation, and emission spectroscopy of terrylene in p -terphenyl: Bulk measurements and singlemolecule studies J. Chem. Phys. 107, 7673 (1997); 10.1063/1.475107 Implementation of a SELFOC lens for the light collection element for single molecule spectroscopy at cryogenictemperatures Rev. Sci. Instrum. 68, 254 (1997); 10.1063/1.1147819

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JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER 19 15 NOVEMBER 1999

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Absorption spectroscopy on single molecules in solidsLothar Kadora)

University of Bayreuth, Institute of Physics and Bayreuther Institut fu¨r Makromoleku¨lforschung (BIMF),D-95440 Bayreuth, Germany

Tatiana Latychevskaia,b) Alois Renn, and Urs P. WildPhysical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zu¨rich,Switzerland

~Received 6 July 1999; accepted 8 September 1999!

Absorption signals of single terrylene molecules inn-hexadecane and naphthalene crystals wererecorded at liquid-helium temperatures. The method is based upon rf Stark effect modulation in themegahertz range. The electric rf field strength was applied by means of interdigitating electrodeswith 18 mm spacing. Signal-to-noise ratios better than 10 were obtained with approximately 300 msintegration time. The measured line shapes depend on the relative contributions of the linear and thequadratic Stark shift. ©1999 American Institute of Physics.@S0021-9606~99!01643-8#

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The spectroscopy of individual dopant molecules in sids at low temperature has proven to be a powerful techniin the field of optical spectroscopy. It has the advantagecompletely eliminating any effects of ensemble averagand, in this way, provides true local information about tinteraction between a dye molecule and its matrix envirment. A large variety of experiments have been performon single molecules; for reviews see Refs. 1–4.

The first spectroscopic investigation of single molecuwas an absorption measurement which used a complicdouble-modulation scheme.5,6 It was based upon opticafrequency-modulation spectroscopy~FMS!7 with modulationfrequenciesnm between 51 and 91 MHz in order to eliminalow-frequency laser noise. FMS experiments are affectedresidual amplitude modulation~RAM! which produces spurious background signals due to unavoidable imbalancestween the amplitudes of the two laser sidebands.8 Hence, asecondary modulation had to be used to eliminate the RATwo different techniques were applied, both of which causa periodic shift of the molecular absorption frequencnamely, ac Stark modulation~modulation frequency,f m

52 – 5 kHz! and ultrasonic modulation~ f m52 – 5 MHz!.With both methods, single-molecule spectra could becorded but the signal-to-noise ratio~SNR! was only abouttwo or three.5,6 Shortly afterwards, the first fluorescencexcitation experiment on single molecules was performwhich provided a much better SNR with a simpler setu9

Therefore, all subsequent experiments used the fluorescexcitation scheme. The dye-matrix system investigatedthese early measurements was pentacene inp-terphenyl.

In the present communication it is demonstrated thatsorption signals of single molecules can also be recorwith good SNR using a simple setup. The basic idea isapply high-frequency modulation—for eliminating lowfrequency laser noise—without the problematic FMS scheso that a secondary modulation is not necessary. We c

a!Electronic mail: lothar.kador@uni-bayreuth.deb!Electronic mail: tatiana@phys.chem.ethz.ch

8750021-9606/99/111(19)/8755/4/$15.00

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fast Stark effect modulation in the rf regime.10 Also ultra-sonic modulation would be possible, as was previously ufor the sensitive detection of hole-burning spectra,11 but inthis case the modulation signal must be gated to preventbuildup of incoherent ultrasound in the transducer andsample. Moreover, in the case of ultrasonic modulationmodulation frequency is determined by the resonancequency of the transducer and cannot be varied duringexperiment. Stark effect modulation of hole-burning specwas reported in Refs. 12 and 13. The modulation frequenwere only in the range between 200 and 300 Hz, howev

The experimental setup is schematically depicted in F1. The light from a tunable single-mode cw dye las~COHERENT CR599-21 operating with rhodamine 6G! wasfocused onto the sample by a microscope objective of hnumerical aperture~Newport M-603; NA50.85!. Thesample was placed on a Pyrex™ chip~537.5 mm2) withevaporated interdigitating gold electrodes of 250 nm thiness. The width and distance between the electrodes weach 18mm. They were covered with a 50-nm-thick protetive layer of silicon carbide~SiC!.14 Contacting was accomplished with a two-component silver epoxy. The sampholder was adjusted so that the laser focus was locatebetween two gold electrodes on the glass chip. The diamof the focus~<1 mm! was much smaller than the electrodspacing. Electrodes, sample, and microscope objective wimmersed in superfluid helium in a bath cryostat~Janis SVT!at a temperature of 1.8 K. Outside the cryostat, an achrolens ~focal length 80 mm! and a camera objective~focallength 58 mm! were used to focus the transmitted, strongdivergent laser light onto a fast avalanche photodiode wintegrated preamplifier~EG&G C30950F; cutoff frequency100 MHz!.

The rf signal for the Stark effect modulation was prvided by a precision rf generator~Marconi 740A! whose out-put was split into two equal parts by a power splitter~MiniCircuits ZSC-2-2!. One part was properly attenuated asubsequently amplified by a wideband rf power amplifi~Kalmus 700LC! to typical amplitudes of 5–7 V peak to

5 © 1999 American Institute of Physics

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peak. This signal was fed into the cryostat and connectethe interdigitating electrodes on the sample holder. Tmodulation frequencies were in the range between 1 andMHz.

The rf component of the photocurrent was amplified blow-noise rf amplifier~Trontech W150D! and demodulatedin a double-balanced mixer~Mini Circuits GRA-6! whichreceived its local-oscillator signal from the second outport of the power splitter. For some of the measurementhigh-pass filter~Mini Circuits SHP-50! was inserted betweethe photodetector and the Trontech amplifier. The output~IF;intermediate-frequency! signal of the mixer was low-pass filtered ~5 MHz! and fed into a custom-built low-frequencamplifier~PMPA; postmixer preamplifier!. The amplifier sig-nal was finally smoothed by a simple RC circuit with adjuable resistor, digitized, and stored in the computer.

Two different matrices were investigated,n-hexadecaneand naphthalene, each doped with terrylene. In the first ca tiny amount of the dye was simply dissolvedn-hexadecane which is liquid at room temperature. The sbility of terrylene inn-hexadecane is extremely low so thsamples with the proper concentration for single-molecexperiments could be prepared. Doped crystal flakes of nathalene were obtained by cosublimation at a temperaturthe educt material of 180 °C.15 The flakes were melted in aoven at about 80 °C. In both cases, a drop of the liquid wplaced on the Pyrex chip and quickly covered with a smpiece of a microscope cover glass. Upon cooling, polycrtalline layers with thicknesses of several microns wformed.

If a single molecule exhibits a linear Stark effect,absorption line is shifted on the frequency axis with the satime dependence~i.e., the same period! as the applied electric rf field. Phase-sensitive demodulation of the photocrent signal in the mixer yields then a line shape whichproportional to the derivative of the molecular absorptiline ~for sufficiently small modulation amplitudes!. This isanalogous to the phase-sensitive ultrasonic modulationhole spectra11 and also to standard experiments in electrparamagnetic resonance~EPR!. If the Stark shift is notpurely linear but has a quadratic contribution, however,demodulated signal is expected to be asymmetric. Moreo

FIG. 1. Schematic plot of the experimental setup. MO—microscope obtive, S—sample, APD—avalanche photodiode, RF—rf generator, PSpower splitter, AMP—rf amplifiers, M—mixer, LPF—5 MHz low-pass fiter, PMPA—postmixer preamplifier, RC—RC circuit.

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in this case the photocurrent will contain a component whoscillates at twice the modulation frequency.

Static Stark effect measurements~with dc fields! onsingle terrylene molecules inp-terphenyl crystals showedthat the Stark shift is very different from molecule to moecule. Most molecules exhibit a linear Stark effect butsome cases no shift was observed at all with the appliedfield strengths of a few kilovolts per centimeter.16 These lat-ter molecules which must be located in highly symmetrienvironments, are expected to show a quadratic Stark eat higher field strengths. Differences in the Stark shift wealso found for terrylene inn-hexadecane and naphthalene17

This has consequences for the observed absorptionshapes.

Figure 2 shows the absorption signal of a single trylene molecule inn-hexadecane. The laser wavelength w572.097 nm, the modulation frequency 1 MHz. The laswas scanned over 2 GHz from lower to higher frequencand then with the same rate back in the reversed directHence, two copies of the absorption line appear in the pThe line shape represents the derivative of a Lorentz prowith equal amplitude above and below the baseline sowe can conclude that this molecule shows a linear Starkfect. Some of the smaller features also appear in both ditions of the laser scan and can be attributed to out-of-fomolecules or molecules with unfavorable orientations. Tsignals at60.6 GHz have a slope of opposite sign; henthe corresponding molecule shows a Stark shift in the opsite direction.

The spectrum was recorded with a laser power ofmW ~measured in front of the cryostat!. The time constant ofthe RC circuit was adjusted to about 150 ms. Two scwere averaged so that the effective time constant wasproximately 300 ms. The resulting SNR is at least 10. Tbackground signal of the baseline is mainly due to an offvoltage produced by the mixer. The strong single-molecsignal has a linewidth of 9566 MHz ~peak to peak!. This ismore than twice as broad as the low-power limit of 4062MHz which was measured for this system.18 The extrabroadening may be due to the modulation amplitude~an ef-fect well-known in EPR experiments! or to optical satura-tion. We made no attempt to eliminate possible broaden

c-rfFIG. 2. Single-molecule absorption signal of terrylene inn-hexadecane at572.097 nm. The direction of the laser scan was reversed at the frequposition 2 GHz~indicated by the dashed straight line!; hence, the two halvesof the plot represent the same spectral range. Shown is the amplifiedsmoothed output~IF! signal of the mixer. Modulation frequency, 1 MHz.

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effects. Yet, for terrylene inn-hexadecane a distribution osingle-molecule linewidths was found which ranges upmore than 100 MHz.19

Figure 3 contains another example of single-molecspectra measured on the system terrylene inn-hexadecaneThe four traces were recorded in the same spectral rang572.097 nm with modulation frequencies of 1 MHz~a!, 10MHz ~b!, 40 MHz ~c!, and 58 MHz~d!. Two single-moleculelines appear in all four scans at60.85 and61.7 GHz. Theyhave again opposite sign and, therefore, opposite Stark sIn contrast to Fig. 2, both signals extend mainly to one sfrom the baseline. We ascribe this to the presence of blinear and quadratic Stark effect contributions. Future expments with different dc bias voltages between the electroand with local-oscillator signals at twice the modulation frquency will help to elucidate and interpret the line shapesdetail. The overall sign change of the signals betweenscans has no physical significance. It is due to a variatiothe mixer offset voltage which made it necessary to chathe polarity of the signals for AD conversion in our eletronic setup. Another origin is the change of the phaseference between the two input signals of the mixer at diffent modulation frequencies.

In addition to the two permanent signals, a third singmolecule line jumps repeatedly in and out at the frequeposition61.85 GHz. Light-driven spectral jumps of part othe molecules were already reported for this system in R18.

The best SNR was obtained with modulation frequencof 10 and 40 MHz@traces~b! and ~c!#. At 1 MHz, someexcess low-frequency noise seems to still be present~a!,whereas at 58 MHz the baseline is affected by time-varyrf pickup ~d!. Absorption signals could be recorded wimodulation frequencies up to about 110 MHz, but aboveMHz the quality of the data decreased due to spuriousignals since we used mainly BNC cables. For future msurements they will be replaced by rf cables with SMA conectors; then the problem should be absent.

Absorption signals of single molecules were also oserved in the system terrylene in naphthalene. An exampshown in Fig. 4. The four traces were recorded at 574.

FIG. 3. Four scans over the same spectral range at 572.097 nm with simolecule modulation signals of terrylene inn-hexadecane. The modulatiofrequencies were 1 MHz~a!, 10 MHz ~b!, 40 MHz ~c!, and 58 MHz~d!. Thescale of the ordinate axis corresponds to spectrum~a!; the other spectra wereshifted vertically for clarity.

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nm with modulation frequencies of 1 MHz~a!, 10 MHz ~b!,20 MHz ~c!, and 40 MHz~d!. The experimental conditionswere the same as for then-hexadecane matrix. The signalof derivativelike shape so that the molecule shows a pdominantly linear Stark effect. Similar to then-hexadecanesystem, molecules with a strong quadratic contribution walso found. The peak-to-peak width of the line in Fig. 410668 MHz.

In summary, we have demonstrated that absorption sptra of single molecules can be recorded at liquid-helium teperatures with good signal-to-noise ratio. The methodbased on rf Stark effect modulation with modulation frquencies in the megahertz range. The electric rf field wgenerated with interdigitating electrodes evaporated ontPyrex chip. The light transmitted through the sample wdetected with a fast avalanche photodiode and demodulawas performed with a double-balanced mixer. Two polycrtalline dye-matrix systems were investigated, terrylenen-hexadecane and in naphthalene. The molecules shodifferent line shapes, which is ascribed to varying contribtions of the linear and the quadratic Stark effect.

Future experiments will show whether absorption signof comparable signal-to-noise ratio can also be recordedother dye molecules which have smaller absorption crsections and/or lower saturation intensities thterrylene.20,21 If so, it is expected that with the new methosingle-molecule spectroscopy can be extended to mclasses of dye molecules. Possible new candidates areecules with low fluorescence quantum yields or molecuand systems which emit mainly resonant rather than rshifted fluorescence photons.

The authors wish to thank P. Nyffeler and R. Weiner fimportant help in the electronic design, H. Bach for tpreparation of the naphthalene samples, and E. Donley fcritical reading of the manuscript. Financial support from tETH Zurich is gratefully acknowledged.

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2L. Kador, Phys. Status Solidi B189, 11 ~1995!.3Single-Molecule Optical Detection, Imaging, and Spectroscopy, edited by

le-FIG. 4. Four scans of a single-molecule modulation signal of terrylenenaphthalene at 574.293 nm. The modulation frequencies were 1 MHz~a!, 10MHz ~b!, 20 MHz ~c!, and 40 MHz~d!. The scale of the ordinate axicorresponds to spectrum~a!; the other spectra were shifted vertically foclarity.

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Th. Basche´, W. E. Moerner, M. Orrit, and U. P. Wild~VCH, Weinheim,1997!.

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~1996!.17H. Bach, A. Renn, G. Zumofen, and U. P. Wild, Phys. Rev. Lett.82, 2195

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