No abstract

No abstract

The size of thermally or photochemically produced colloids is controlled by their Mie absorption band. Excitation in this band shows strong resonant SERS. Vibrational contributions contain one-phonon scattering of the host and localized (or resonant) modes due to a cation-silver bonding. The continuous background only extends t o the Stokes side with a cut-off frequency influenced by the exciting photon energy. The anti-Stokes side has its cut-off a t = 0 with a temperature-dependent shape given by the Fermi factor of the metal electrons.

The continuum background of the SERS spectrum from thermally or photochemically prepared Ag colloids in alkali halide crystals is studied from − 5000 to + 1400 cm−1 and for counting rates covering three decades. The Stokes side consists of three contributions extending to about − 1500, − 3800, and beyond − 5000 cm−1 for 488 nm exciting light. As long as there is no evidence for hot luminescence it may be interpreted as SERS caused by charge‐transfer from the Fermi sea of the silver to mobility states and band‐tail states of the host and to localized surface states of the colloid. The anti‐Stokes intensity of the continuum background is found to follow a Boltzmann law between room temperature and 80 K.

Thermally and photochemically prepared Ag colloids in various alkali halides show a low-frequency mode (LFM) between 3 and 8 cm-'. By stepwise annealing the crystal the LFM band maximum is shifted to lower wave numbers. The average size of the Ag colloids is determined from the LFM spectrum. The independence of the LFM on the polarization of the exciting laser light indicates predominantly spherically shaped Ag colloids. The influence of temperature on the LFM is investigated both, on Stokes and anti-Stokes side. After dividing the LFM intensities by the Bose function the reduced SERS signal is found to be temperature independent and mirror symmetric concerning the Stokes shift.Thermisch und photochemisch praparierte Silberkolloide in verschiedenen Alkalihalogeniden zeigen eine ,,Low-frequency mode (LFM)" zwischen 3 und 8 cm-'. Durch schrittweises Tempern des Kristalls verschiebt sich das Bandenmaximum der LFM zu kleineren Wellenzahlen. Die mittlere GroDe der Silberkolloide wird aus dem Spektrum der LFM ermittelt. Die Unabhangigkeit der LFM von der Polarisation des anregenden Laserlichtes laBt auf Silberkolloide mit uberwiegend spharischer Gestalt schlieBen. Der EinfluB der Temperatur auf die LFM wird fur die Stokes-und Anti-Stokes-Seite untersucht. Nach Division der LFM-Intensitaten mit der Bose-Funktion ergibt sich ein reduziertes SERS-Spektrum, das von der Temperatur unabhangig und symmetrisch bezuglich der Stokes-Verschiebung ist. ') Wilhelm-Klemm Str. 10, D-4400 Miinster, FRG. *) Part of thesis work, Munster 1989.