carotenoids are widespread in nature and 
 have important functions 
 in plants for light harvesting and for protection against excess light . in humans and other mammals , carotenoids , especially 
 -carotene , serve as precursors for vitamin a , which is a part 
 of photoreceptors for the visual process . the 
 versatile natural functions and the underlying structural properties 
 of carotenoids , particularly the large -electron system generating 
 energetically low excited states , make 
 them interesting objects for spectroscopic studies . resonance raman spectroscopy has been extensively used 
 to characterize -carotene as well as the influence of the different molecular 
 structures of many carotenoids on their vibrational spectra . on the basis of this fundamental knowledge , raman spectroscopy , 
 both in- and off - resonance with the    * electronic 
 transition in the molecules , has been suggested as a tool to investigate 
 the composition of carotenoid components and their behavior in naturally 
 occurring matrices , such as plants and lipid membranes . in this context , hyper - raman scattering can be useful , as it provides 
 the possibility to combine near - infrared two - photon excitation , which 
 has little impact on biological samples , with resonance - enhanced scattering 
 at the second harmonic of this excitation in the visible . hyper - raman 
 microspectra of -carotene crystals 
 have been reported , and an influence of -carotene on the hyper - raman 
 signals of surrounding solvent molecules has been discussed . under truly nonresonant conditions , measurements 
 in dilute solutions are desirable , because when solid samples or solutions of carotenoid 
 molecules at high concentration are investigated the strong electronic 
 transition of carotenoids can also qualitatively influence their raman 
 spectra upon excitation in the near - infrared , far away from resonance . however , in contrast to the resonant case excited 
 in the visible , inherent raman scattering 
 cross sections upon excitation in the near - infrared are too small 
 to obtain spectra at low concentration . plasmonic nanostructures can 
 increase the signal intensities of raman scattering by several orders 
 of magnitude in surface - enhanced raman scattering ( sers ) and surface - enhanced 
 hyper - raman scattering ( sehrs ) , thus 
 improving sensitivity in dilute solutions . furthermore , sers and especially 
 sehrs spectra are influenced by the presence of the metal surface , 
 such as by the adsorption geometry , and by the changing chemical environment . in particular , for dyes with absorption maxima close to the plasmon 
 band of the metal nanostructures , the interaction between the molecules 
 and the metal additionally results in changes of the electronic properties . -carotene can also take part in such a coupling when it is 
 placed close to a silver surface . here , 
 we use sers and sehrs under molecular resonant and also nonresonant 
 conditions to characterize the interaction between carotenoid molecules 
 and silver nanoparticles . the adsorption of carotenoids to silver 
 nanoparticles in aqueous 
 solution is only straightforward for carotenoids with hydrophilic 
 end groups . -carotene and other 
 nonpolar carotenoids are poorly soluble in water and have low affinity 
 to the silver surface . in earlier work , to obtain sers spectra , the 
 affinity between the silver surface and the carotenoids was increased 
 by functionalization of the silver nanostructures with organic layers . in the work reported here , we increase the affinity between carotene 
 and silver nanoparticles by using 7-apo-7-(4-mercaptomethylphenyl)--carotene , 
 which was shown to adsorb to gold surfaces via its thiol group . as organothiols in general strongly chemisorb 
 to silver surfaces in a similar way as to gold , we expected a similar interaction between this functionalized carotene 
 and silver nanoparticles . to investigate this , we compare the resonant 
 sers and sehrs spectra of the thiol - functionalized to those of the 
 normal , nonfunctionalized -carotene ( for a comparison of the 
 molecular structures , see figure 1a ) . as will be demonstrated , the stronger interaction 
 between the functionalized carotene and the silver nanoparticles enables 
 the investigation of the nonresonant sers spectrum of carotene . ( a ) molecular 
 structures of the two carotenoids examined in this 
 study : 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 ( 1 ) and -carotene ( 2 ) . ( b ) absorbance 
 spectra of the silver nanoaggregates before and after the addition 
 of carotene solutions with the same concentrations as in the sehrs 
 experiments . the inset in ( b ) shows a transmission electron micrograph 
 of the silver nanoparticles ( scale bar : 100 nm ) . silver nanoparticles with an average 
 diameter of 42 nm were produced 
 by the reduction of silver nitrate with hydroxylamine hydrochloride , resulting in an aqueous solution with a particle 
 concentration of 7  10 m. the carotenethiol 
 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 was synthesized following a procedure reported by gust et al . and leatherman et al . the final product and all of the intermediates were stored under 
 argon at 20 c , and reaction mixtures and products were 
 shielded from light . h nmr spectroscopy confirmed the formation of 7-apo-7-(4-mercaptomethylphenyl)--carotene . 
 for determining the actual amount of the carotenethiol , absorbance 
 spectra of a defined solution of the synthesis product in dichloromethane 
 were measured . from these spectra , the concentration of the carotenethiol 
 in the measured solution , and thus its proportion in the synthesized 
 product , solutions of 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 and of nonfunctionalized -carotene ( fluka , > 97% ) in ethanol 
 of different concentrations were added to the nanoparticles and nanoaggregates 
 in a volume ratio of 11:1 ( aqueous nanoparticle solution / carotene 
 solution ) to yield the different final carotene concentrations ( described 
 in the respective figure captions ) . samples for sehrs experiments , 
 and also for accompanying sers experiments with 532 nm excitation 
 ( figure 5 ) , were prepared 
 in the same way but were finally diluted with 10% ( v / v ) aqueous ethanol to obtain a ten times lower nanoparticle concentration 
 than in the other experiments and a final carotene concentration of 
 3  10 m. hyper - raman spectra and near - infrared 
 excited raman spectra were 
 excited at 1064 nm with a mode - locked laser producing 7 ps pulses 
 with a repetition rate of 76 mhz . the peak photon flux density at 
 the sample was between 3  10 and 6  10 photons cm s ( average 
 power at the sample ranging from 50 to 500 mw ) . excitation at 532 
 nm was provided by a cw laser with a photon flux density between 1 
  10 and 6  10 photons cm s ( average power ranging from 
 2 to 10 mw ) at the sample . the scattered light was collected 
 in backscattering geometry , dispersed in a grating spectrograph , and 
 detected by a liquid nitrogen - cooled ccd ( hyper - raman spectra and 
 532 nm excited raman spectra ) or a liquid nitrogen - cooled ingaas detector 
 ( 1064 nm excited raman spectra ) . for sers experiments with 514.5 nm 
 excitation , a labram hr800 ( horiba jobin - yvon ) setup coupled to a 
 microscope and equipped with an argon ion laser was used . this setup 
 was used because it enables the observation of relatively low raman 
 shifts in the range 200250 cm . the spectral 
 resolution in the full spectral range was 23 cm for the 1064 nm excited sers and raman spectra , 68 cm for the hyper - raman spectra , sehrs spectra , and 532 
 nm excited sers and raman spectra of solutions , 913 cm for the 532 nm excited raman spectra of solid samples , 
 and 1115 cm for the 514.5 nm excited sers 
 spectra . the sehrs and near - infrared excited sers spectra were 
 baseline 
 corrected using a method developed by zhang et al . the surface - enhanced 
 raman and surface - enhanced hyper - raman spectra of the thiol - functionalized 
 carotene 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 ( 1 in figure 1a ) and nonfunctionalized -carotene ( 2 in figure 1a ) were obtained 
 with colloidal silver nanoparticles using visible and near - infrared 
 excitation wavelengths . the enhancing plasmonic nanostructures are 
 silver nanoaggregates in water produced by the reduction of silver 
 nitrate with hydroxylamine , which are well - known for producing high 
 sers enhancement . the broadening of the absorbance 
 band of the silver nanostructures in the presence of the carotene 
 solutions ( figure 1b ) indicates the presence of nanoaggregates due to an interaction 
 between the molecules and the metal , and superimposes the molecular 
 absorption of the carotene molecules . we expected the carotenethiol 
 to chemisorb to the silver nanoparticles via the thiol group similar 
 to previous experiments reported with the same carotenethiol on a 
 gold surface . however , different from 
 the previous work with the molecule , we 
 use here an aqueous solution instead of nonpolar organic solvents , 
 which could have an impact on the interaction of the carotene molecules 
 with the metal surface . as an example , in the presence of water , molecular 
 aggregates of the molecules can form . figure 2a shows 
 the nonresonant sers spectrum of the carotenethiol excited at 1064 
 nm . the most prominent bands in this spectrum are those at 1520 , 1155 , 
 and 1005 cm , corresponding to the c = c stretching , 
 the c c stretching , and the methyl - rocking vibrations , respectively , 
 of the polyene chain , which are well - known from resonant raman and 
 resonant sers ( serrs ) spectra of -carotene . the sers spectra obtained off - resonance , 
 however , differ in the intensity ratios of these bands and the nonresonance - enhanced 
 ones at 1591 and 1178 cm from the resonant raman 
 and serrs spectra of -carotene . as will be discussed below , 
 this may indicate a very different sers enhancement in the case of 
 resonant and nonresonant raman signatures . ( a ) sers spectrum of 
 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 and ( b ) comparison between the sers spectra of 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 ( upper trace ) and nonfunctionalized -carotene ( lower trace ) . 
 final concentration of the carotenes in the sample solutions : 3  
 10 m ; excitation : 1064 nm ; peak photon flux density : 
 3  10 photons cm s ; and acquisition time : ( a ) 30 s and ( b ) 120 s. for the spectrum 
 in ( a ) , a total of 100 spectra from five different samples were averaged 
 after baseline correction . as illustrated in figure 2b , compared to the carotenethiol spectrum ( black trace ) , 
 the 
 near - infrared excited sers spectrum of the nonfunctionalized carotene 
 ( blue trace ) , shows only very weak signals even at integration times 
 of a few minutes . the band at 1155 cm in the spectrum 
 of carotenethiol in figure 2b ( black trace ) is nine times more intense than the same band 
 in the spectrum of the nonfunctionalized carotene ( blue trace in figure 2b ) , even though both 
 spectra were acquired under the same experimental conditions . first , the thiol group could cause a higher affinity between the functionalized 
 carotene and the silver surface , resulting in a higher sers enhancement 
 for the thiolated carotene due to a smaller molecule  metal 
 distance . second , the modification of the molecular structure also 
 includes the enlargement of the mesomeric system by an additional 
 phenyl ring ( figure 1a ) ; this could increase the raman scattering cross section , which 
 would affect both sers and normal raman spectra . the quantification 
 of the enhancement in the sers spectra of the 
 two carotenes is interesting when discussing differences in the interaction 
 with the silver surface . as it was not possible to quantify the surface 
 enhancement in the sers spectra excited at 1064 nm because , off - resonance , 
 no reference raman spectra of carotenoid solutions without silver 
 nanoparticles are obtained , we estimate surface - enhancement factors 
 for the two carotenes from serrs and rr spectra that were excited 
 at 532 nm ( figure 3 ) . this excitation wavelength is in resonance with the strongest 
 electronic transition of the carotenes with an absorption maximum 
 between 450 and 480 nm . the serrs spectra in figure 3a show bands with 
 similar intensity for the two carotenes even though the concentration 
 of the nonfunctionalized carotene ( figure 3a , lower trace ) is 1 order of magnitude higher 
 than that of the thiol - modified one ( figure 3a , upper trace ) . this is in accordance with 
 the observation of the weaker signals from the nonfunctionalized carotene 
 for the nonresonant case ( figure 2b ) . by comparing the respective intensities of the 
 bands at 1520 and 1156 cm in the serrs and rr 
 spectra ( figure 3 ) 
 and taking into account the different concentrations , the sers enhancement 
 factors were estimated to be 2  10 for the functionalized 
 and 4  10 for the nonfunctionalized carotene . this 
 relatively low sers enhancement would be in agreement with previous 
 observations of a diminished surface enhancement in some cases of 
 strong molecular resonance , and furthermore , indicates 
 a weak interaction between the molecules and the silver surface for 
 both carotenes . nevertheless , the enhancement is 1 order of magnitude 
 larger for the carotenethiol than for the nonfunctionalized carotene . therefore , there must be a difference in the interaction of the two 
 carotenes with the silver nanoparticles , which can include a higher 
 tendency of the carotenethiol to adsorb to the silver surface . ( a ) serrs spectra 
 and ( b ) resonant raman spectra of 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 ( upper spectra ) and -carotene ( lower spectra ) . concentrations : 
 ( a ) upper trace 3  10 m ; lower trace 3  
 10 m ; ( b ) both traces 3  10 m. excitation : 532 nm ; 6  10 photons cm s ; 1 s acquisition time . bands marked with an 
 asterisk are due to the ethanol in the solvent . to characterize the kind of interaction between the carotene 
 molecules 
 and the silver surface , we examined the region of low raman shifts 
 in 514 nm excited sers spectra ( figure 4 ) . chemical bonds between the silver and adsorbed molecules 
 can be observed in sers spectra by the appearance of modes between 
 200 and 250 cm related to stretching vibrations 
 between the molecules and the silver surface . as shown in figure 4 ( green spectrum ) , the silver nanostructures 
 used in this study exhibit a band at 235 cm , which 
 can be explained by surface - bound chloride or hydroxylamine from the 
 synthesis of the silver nanoparticles . upon addition of carotene solutions , 
 the band at 235 cm strongly decreases ( figure 4 , blue spectrum ) , 
 and for the thiolated carotene , the band disappears completely ( figure 4 , black spectrum ) . this suggests that the adsorbates , which are present on the silver 
 surface from the synthesis of the nanoparticles , are exchanged by 
 carotene molecules . the exchange seems to be more complete for the 
 carotenethiol than for the nonfunctionalized carotene , as is indicated 
 by the small band at 235 cm remaining in the sers 
 spectrum of the nonfunctionalized carotene ( figure 4 , blue spectrum ) . considering the high affinity 
 between sulfur and silver , and also because the 
 carotenethiol was reported to adsorb on gold surfaces via the thiol 
 group , formation of an ag it is possible that such a bond has formed 
 without being detected in the serrs spectra because the ag s 
 stretching band would not be resonance enhanced in contrast to the 
 carotene bands . on the other hand , the absence of a band in the 200250 
 cm region in the sers spectrum of the carotenethiol 
 could also indicate that no chemical bond between the thiol group 
 of the carotenethiol and the silver surface is formed at all . in this 
 case , the interaction between the carotenethiol and the silver surface 
 would happen via the -electrons in the polyene chain . however , 
 this interaction would be weaker than a chemisorption of the thiol 
 group and would take place in a similar fashion for the nonfunctionalized 
 carotene . in such a case , the higher enhancement in the sers spectra 
 of the functionalized carotene would result from the larger part of 
 the molecule interacting with the silver nanostructure . sers spectra 
 of the carotenethiol 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 ( 3  10 m , upper trace ) and the nonfunctionalized 
 -carotene ( 10 m , lower trace ) with the 
 spectrum from silver nanoaggregates without carotene molecules . the 
 spectrum of the nonfunctionalized carotene was scaled up by a factor 
 of 3 to facilitate comparison . excitation : 514.5 nm , 2  10 photons cm s , and 
 1 s acquisition time . to characterize the molecules 
 at the nanoparticle surfaces , we also discuss their surface - enhanced 
 hyper - raman ( sehrs ) spectra . figure 5a shows the sehrs 
 spectra of the carotenethiol and of nonfunctionalized carotene obtained 
 at an excitation wavelength of 1064 nm . hyper - raman scattering , as 
 a nonlinear process , follows symmetry selection rules different from 
 those of linear raman scattering , especially 
 for centrosymmetric molecules where the allowed modes in the two processes 
 are mutually exclusive . upon adsorption 
 to a metal surface , molecular symmetry can be altered such that sers spectra of a chemical compound can be different 
 from the corresponding raman spectra . this effect is even more pronounced for sehrs , which has been observed 
 to strongly depend on the orientation of a molecule on the metal surface 
 and on the chemical environment . additionally , as the second harmonic of the wavelength that we use 
 to excite sehrs spectra is close to the electronic transition of carotene , 
 a contribution of resonance enhancement can be expected . ( a ) 
 sehrs spectra excited at 1064 nm and ( b ) serrs spectra excited 
 at 532 nm of 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 ( upper spectra in both panels ) and of the nonfunctionalized -carotene 
 ( lower spectra in both panels ) . final concentrations of the carotenes 
 in the sample solutions : 3  10 m ; excitation : 
 ( a ) 1064 nm ; peak photon flux density 6  10 photons 
 cm s ; 120 s acquisition time ; 
 ( b ) 532 nm ; 7  10 photons cm s ; 100 ms acquisition time . bands marked with an 
 asterisk in ( b ) are due to the ethanol in the solvent . the exchange of one of the carotene end groups 
 when functionalizing 
 the -carotene should cause qualitative changes in the sehrs 
 spectra . nevertheless , the spectra of the two carotenes are similar 
 ( figure 5a ) except 
 for a few details : the bands at 1190 and 1271 cm in the spectrum of the nonfunctionalized carotene ( lower spectrum 
 in figure 5a ) are shifted 
 to 1173 and 1258 cm , respectively , in the spectrum 
 of the carotenethiol ( upper spectrum in figure 5a ) . the same is observed for the serrs spectra , 
 which were obtained quasi - simultaneously from the same samples as 
 the sehrs spectra ( figure 5b ) . in the spectra of nonfunctionalized -carotene , 
 the two bands are assigned to c  c stretching and in - plane c - h 
 bending vibrations , respectively , in the polyene chain . the shift of these two bands to 1173 and 1258 
 cm , respectively , in the spectra of the carotenethiol 
 can be explained by an influence on the mesomeric system of the polyene 
 chain by the insertion of the phenyl ring in the carotenethiol ( 1 in figure 1a ) . the two frequency shifts are observed in the normal raman and 
 normal hyper - raman spectra of powder samples of the two carotenes 
 as well ( figure 6 ) . therefore , we attribute them to the different structures of the molecules 
 as such and not to changes in the interaction with the silver surface . ( a ) nonresonant 
 raman spectra , ( b ) hyper - raman spectra , and ( c ) 
 resonant raman spectra of 7-apo-7-(4-mercaptomethylphenyl)--carotene 
 ( upper spectra ) and of the nonfunctionalized -carotene ( lower 
 spectra ) measured from solid samples . excitation : ( a ) 1064 nm ; peak 
 photon flux density 3  10 photons cm s ; acquisition time 20 s ; ( b ) 1064 nm ; peak 
 photon flux density 1  10 photons cm s ; acquisition time 600 s ; ( c ) 532 nm ; peak 
 photon flux density 1  10 photons cm s ( upper spectrum ) ; 5  10 photons cm s ( lower spectrum ) ; 
 1 s acquisition time . while normal raman and hyper - raman spectra of carotene molecules 
 differ greatly , there is a high similarity 
 of the sehrs and serrs spectra because , due to the interaction with 
 the silver surface , the symmetry of the molecules is lowered . nevertheless , 
 two differences can be observed between the respective one- and two - photon 
 excited spectrum in both carotenes ( figure 5 ) : ( i ) a shoulder band at 1051 cm appears in the sehrs spectra ( figure 5a ) , which is absent in the serrs spectra ( figure 5b ) . at the same position , a band in the infrared absorption spectrum of -carotene has 
 been reported , which is due to a vibration of the ionone ring and 
 which is not observed in one - photon raman spectra . the appearance of this band in our sehrs spectra is in 
 accordance with the fact that the symmetry selection rules for two - photon 
 excited raman spectra are usually expected to be more similar to those 
 for ir - spectra than to those for one - photon excited raman spectra . ( ii ) the weak band at 1591 cm in the serrs spectra ( figure 5b ) that is quite obvious in the nonresonant sers spectra of 
 the carotenethiol ( figure 2a ) appears as a shoulder next to the band at 1520 cm instead of as a separate band in the sehrs spectra ( figure 5a ) . it seems that , in the sehrs 
 spectra , this band is either shifted to a lower raman shift value 
 or contains contributions from an additional vibration at lower frequency . the band at 1591 cm can be assigned to a c = c 
 stretching vibration of a double bond at the end of the polyene chain 
 and has been observed to coexist with a weaker band of another c = c 
 stretching vibration at around 1580 cm . it is possible that the band around 1580 cm is selectively enhanced in the sehrs spectra but 
 is too weak to be observed in one - photon excited sers spectra due 
 to different surface selection rules in the two excitation regimes . an additional contribution to the shoulder at 1591 cm can be made by an ir - active but raman - inactive c = c stretching 
 vibration , which was previously observed as an intense band at 1569 
 cm in hyper - raman spectra of -carotene 
 upon excitation at shorter wavelengths , together with additional ir - active 
 modes . the presence of such a contribution from 
 the ir - active c = c stretching vibration is suggested by the 
 second derivative of the sehrs spectrum ( data not shown here ) and 
 is also supported by the hyper - raman spectra of solid nonfunctionalized 
 -carotene , where we observe a small band at 1559 cm ( figure 6b , lower 
 trace ) . comparison of the serrs ( figure 5b ) and sehrs spectra ( figure 5a ) shows relatively strong contributions 
 in the sehrs spectra from bands that experience a high sers enhancement 
 but low resonance enhancement . specifically , the band at 1591 cm is more intense than the band in the serrs spectra 
 ( figure 5b ) . in a similar 
 fashion , the relative intensity of the band at 1178 cm shows high relative intensity in the sehrs spectrum of the carotenethiol 
 ( figure 5a , upper trace ) . conversely , describing this as a relative decrease in intensity of 
 the bands at 1520 and 1155 cm that experience 
 strong resonance enhancement , the sehrs data ( figure 5a ) would support the theoretical consideration 
 and serrs experimental observation of decreased molecular resonance 
 enhancement due to electromagnetic and/or electronic coupling of the 
 molecules and the metal nanoparticles . in the 
 nonresonant sers spectrum of the carotenethiol ( figure 2a ) , the band at 1591 cm is more intense than in the sehrs ( figure 5a ) and serrs ( figure 5b ) spectra , as seen by the intensity ratio 
 between this band and the band at 1520 cm . a similar 
 change in intensity ratio is observed for the bands at 1178 and 1155 
 cm in the near - infrared excited sers spectrum 
 ( figure 2a ) compared 
 to the serrs ( figure 5b ) and sehrs ( figure 5a ) spectra . the bands at 1520 and 1155 cm , which 
 represent c = c and c  c stretching vibrations , respectively , 
 in the center of the polyene chain experience the largest influence 
 by resonance enhancement . therefore , the 
 reason for the different intensity ratios between these bands and 
 those at 1591 and 1178 cm , which are due to c = c 
 and c  c vibrations at the end of the polyene chain in the spectra 
 at visible and near - infrared excitation is the absence of resonance 
 enhancement in the near - infrared excited spectra . these intensity 
 changes are only observed in the sers spectra , which are obtained 
 from carotene in silver nanoparticle solutions but not in the nonresonant 
 or rr spectra of the solid samples ( figure 6a and c ) . this can be explained by -electron  phonon - coupling 
 in the solid samples leading to an influence of the strong electronic 
 transition in -carotene and similar polyenes also on raman 
 spectra excited far away from resonance . the functionalization of carotene , which leads to a stronger interaction 
 with the silver surface , thus gives the possibility to obtain nonresonant 
 sers spectra of carotene molecules on silver nanostructures in solutions 
 at a low concentration of the molecules , where the influence of the 
 rr contribution is decreased . the experiments at the different excitation 
 wavelengths enable a discussion of the superposition of the molecular 
 resonant raman enhancement and of the sers enhancement . in general , 
 similarity between the hrs and rrs spectra ( figure 6b and c , respectively ) 
 and the sehrs and serrs spectra ( figure 5a and b ) , respectively , also indicates that 
 hyper - raman scattering benefits from electronic resonances due to 
 2-photon excitation , and the same electronic transition is responsible 
 in both one- and two - photon excitation . nevertheless , this similarity of the respective one- and two - photon 
 excited spectra differs for the sers ( figure 5 ) and  non - sers  ( figure 6b and c ) cases : we 
 observe high similarity of the sehrs ( figure 5a ) and serrs ( figure 5b ) spectra because the interaction with the 
 silver surface lowers the symmetry of the adsorbed molecules . in contrast , 
 the hyper - raman spectra ( figure 6b ) and rrs spectra of the solid samples ( figure 6c ) differ greatly due to the 
 different selection rules that become very evident for molecules with 
 high symmetry . other studies , working with lower excitation wavelengths , have reported even greater dissimilarities of 
 hrs and rrs spectra of -carotene . in sehrs , similar to previous 
 discussion of serrs , the role of such 
 different contributions from molecular resonance effects that act 
 in addition to the altered symmetry of the adsorbed molecules will 
 have to be studied in excitation profiles in the future . in conclusion , -carotene modified with a thiol group was 
 used to obtain sers and sehrs spectra at 1064 nm excitation as well 
 as serrs spectra at 532 nm excitation using silver nanoparticles . the functionalization leads to a strengthening of the interaction 
 between carotene and the surface of silver nanoparticles , which is 
 seen by higher sers and serrs signals . the improved interaction 
 with the silver surface of the thiol - modified 
 molecules can be utilized to obtain nonresonant sers spectra of carotene 
 in dilute aqueous solution , which show a different intensity pattern 
 than normal raman spectra of solid samples or solutions of high concentration . comparison of the nonresonant sers , resonant sehrs , and serrs spectra 
 suggests that the resonance enhancement plays an important role in 
 the 1064 nm excited sehrs spectra . the sehrs spectra of the thiol - modified 
 and the nonfunctionalized carotene are very similar . in addition to 
 bands that are also found in the serrs spectra , they show several 
 contributions from infrared - active vibrational modes of the polyene 
 chain . relatively strong contributions in the sehrs spectra by bands 
 with low resonance enhancement point to an influence of the electromagnetic 
 and/or electronic coupling of the molecules and the silver nanoparticles 
 on the enhancement in the resonant hyper - raman process . despite these 
 small differences , the sehrs and serrs spectra are highly similar , 
 as opposed to the more severe differences between the hyper - raman 
 spectra and resonant raman spectra of carotenes in solids or in organic 
 solvents . this is in accordance with the assumption that the symmetry 
 of the molecules is lowered upon adsorption to the silver nanoparticles .
a thiol - modified 
 carotene , 7-apo-7-(4-mercaptomethylphenyl)--carotene , 
 was used to obtain nonresonant surface - enhanced raman scattering ( sers ) 
 spectra of carotene at an excitation wavelength of 1064 nm , which 
 were compared with resonant sers spectra at an excitation wavelength 
 of 532 nm . 
 these spectra and surface - enhanced hyper - raman scattering 
 ( sehrs ) spectra of the functionalized carotene were compared with 
 the spectra of nonmodified -carotene . using sers , normal raman , 
 and sehrs spectra , 
 all obtained for the resonant case , the interaction 
 of the carotene molecules with silver nanoparticles , as well as the 
 influence of the resonance enhancement and the sers enhancement on 
 the spectra , were investigated . 
 the interaction with the silver surface 
 occurs for both functionalized and nonfunctionalized -carotene , 
 but only the stronger functionalization - induced interaction enables 
 the acquisition of nonresonant sers spectra of -carotene at 
 low concentrations . 
 the resonant sehrs and sers spectra are very similar . 
 nevertheless , the sehrs spectra contain additional bands of infrared - active 
 modes of carotene . 
 increased contributions from bands that experience 
 low resonance enhancement point to a strong interaction between silver 
 nanoparticles and electronic levels of the molecules , thereby giving 
 rise to a decrease in the resonance enhancement in sers and sehrs .