ISSN 0030-400X, Optics and Spectroscopy, 2009, Vol. 107, No. 4, pp. 505–514. © Pleiades Publishing, Ltd., 2009. Original Russian Text © A.V. Borodin, V.Ya. Gayvoronsky, O.D. Kachkovsky, Ya.A. Prostota, A.V. Kargovskiі, M.M. Nazarov, D.A. Sapozhnikov, Yu.L. Slominskiі, I.N. Smirnova, A.P. Shkurinov, 2009, published in Optika i Spektroskopiya, 2009, Vol. 107, No. 4, pp. 535–545. 505 The terahertz (THz) frequency range is intermedi- ate between the IR and microwave ranges (1 THz 1 ps 300 µm 33 cm –1 4.1 meV 47.6 K). Until recently, the active application of this radiation was limited to a great extent by the absence of convenient effective sources and low-noise detec- tors, which have only been designed in laboratories in the last decade due to the development of femtosec- ond laser sources [1]. Along with other promising applications, THz radiation is used in molecular spec- troscopy [2, 3]. In contrast to the visible and near-IR ranges, where the characteristic frequencies of local- ized vibrations of individual chemical bonds of mole- cules and their electronic transitions lie, the THz spectral response of molecular systems yields informa- tion about slow molecular motions [2] related to the molecules as a whole, their rotations, and phonon vibrations of molecular crystal lattice [4]. In this paper, we report the results of studying the polyatomic molecules aligned as a molecular crystal. The lattice long-range order forms the phonon sub- system of the crystal, which makes a significant contri- bution to the effective THz spectral response. Mole- cules in these crystals are bound by noncovalent inter- molecular (electrostatic, van-der-Waals or hydrogen) bonds. In the initial approximation, the low energy of these bonds compared to that of intramolecular cova- lent bonds allows one to neglect the contribution of intermolecular interaction forces when describing the properties of such a molecular ensemble while con- serving the long-range molecular order as a whole. This approximation is known as the oriented molecu- lar gas model, which well describes the optical proper- ties of molecular crystals [5]. However, as a result of the consideration of the interaction between the ensemble elements, the frequency vibrational spec- trum of the ensemble and each its element is deter- mined by the system as a whole [6]. Even in the 1940s, it was shown that the excitation of lattice phonon vibrations in molecular crystals increases the intensity of intramolecular vibrations, which are correlated to lattice vibrations [7, 8]. The above-mentioned approaches are applicable for THz absorption spectra that contain simultaneously intra- and intermolecular motions [9–11]. Thus, the absorption spectra and the refractive index dispersion in the THz range should be sensitive to changes in the structure of molecular crys- tal and the chemical structure of its molecules. More- over, it was experimentally verified in [3, 12] that molecular crystals of certain symmetries make it pos- sible to strictly select the normal modes related to both the motions of lattice as a whole (phonon mode) and purely intramolecular vibrations. When strong mode–mode coupling is taken into account [13], the experimentally observed features in the THz absorption spectra should be related to the lattice and intramolecular vibrations. For π-conju- gated molecular systems (subject of this study), one might expect THz absorption spectra to be sensitive to the length of π-conjugated chain, properties of donor and acceptor peripheral groups, and the molecular microenvironment. Among the large number of types FUNDAMENTAL PROBLEMS OF OPTICS—2008: TERAHERTZ OPTICS AND SPECTROSCOPY Structure-Sensitive Changes in the Terahertz Absorption Spectra of Merocyanine Dye Derivatives A. V. Borodin a , V. Ya. Gayvoronsky b , O. D. Kachkovsky c , Ya. A. Prostota c , A. V. Kargovskiі a , M. M. Nazarov a , D. A. Sapozhnikov a , Yu. L. Slominskiі c , I. N. Smirnova a , and A. P. Shkurinov a a Physical Faculty and International Laser Center, Moscow State University, Moscow, 119992 Russia b Institute of Physics, National Academy of Sciences of Ukraine, Kiev, 03680 Ukraine c Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Kiev, 03094 Ukraine e-mail: alex@lasmed.phys.msu.ru Received May 12, 2009 Abstract—The low-frequency vibrational modes of the series of merocyanines (malononitrile derivative) have been investigated by pulsed terahertz spectroscopy. The terahertz absorption spectra are shown to con- tain both intermolecular and intramolecular vibrational modes in the range of 0.15–3.45 THz (5–115 cm –1 ). An unambiguous correlation is established between the purposeful modification of the molecular structure of merocyanine dyes and the change in their terahertz absorption spectra. PACS numbers: 33.20.-t DOI: 10.1134/S0030400X09100026