Papers by Saulius Pakalnis
Lithuanian physics journal, 1998
LEOS 2001. 14th Annual Meeting of the IEEE Lasers and Electro-Optics Society (Cat. No.01CH37242), 2001
Shen and co-workers have pioneered sum-frequency spectroscopy and its use to probe interfaces and... more Shen and co-workers have pioneered sum-frequency spectroscopy and its use to probe interfaces and thin films.' In this talk we will we highlight the unique attributes of sum-frequency spectroscopy by reviewing two case studies performed in our laboratory to investigate the orientation of macromolecules at interfaces. The first case study illustrates how spectral information of sum-frequency spectroscopy can be used to probe the orientation of monolayers and also independently probe the anisotropy of a metal interface, which influences the orientation of the monolayers, buried below a monolayer. The second case study illustrates how sum-frequency spectroscopy can be used to obtain spatial information of the uniformity of distribution and the density of coverage of synthetic proteins that are deposited as thin films.
Physical review. B, Condensed matter, Jan 15, 1994
Photoinduced absorption data for polyaniline films in two forms, emeraldine base (EB) and emerald... more Photoinduced absorption data for polyaniline films in two forms, emeraldine base (EB) and emeraldine salt (ES), are presented and discussed. Electrochemically synthesized films were excited with laser pulses of large intensity (the energy =2 eV, the pulse duration = 8 ps, the pulse energy up to 10 mJ/cm'). Optical spectra of the EB and ES films in the temperature range from 298 to 363 K are also studied. The polymer film is presumed to be a three-dimensional (3D) system of long finite conjugated fragments of polymer chains. Considering each fragment integrally and taking into account the electronic polarization energy ( =1.5 eV) for a charge in the film, the 3D model of a charge-transfer exciton is proposed. The low-energy absorption of EB and ES and the electronc processes in photoexcited films are interpreted within the framework of the proposed model, avoiding the commonly adopted approach of an isolated polymer chain.
Lithuanian Journal of Physics, 2005
Oil and water fluorescence properties were studied under their excitation by 266 and 355 nm light... more Oil and water fluorescence properties were studied under their excitation by 266 and 355 nm light, corresponding to the fourth and third harmonics of a Nd:YAG laser.
The Journal of Physical Chemistry B, 2003
The first series of conjugated organic polymers with the formula {1,4-[2-(RO)-5-(R′O)C 6 H 2 ]sNd... more The first series of conjugated organic polymers with the formula {1,4-[2-(RO)-5-(R′O)C 6 H 2 ]sNdNsC 6 H 4 s CtC-} n , where R ) methyl, R′ ) hexyl (7a) and R ) hexyl, R′ ) SiMe 2 -t-Bu (8), were produced in palladium-catalyzed cross-coupling and homocoupling reactions of selected bis(bromo) and bis(alkynyl) azo monomers. The molecular weights (M n ) for the conjugated polymers ranged from 5500 to 22 000. Degenerate four wave mixing (DFWM) with 780 nm femtosecond pulses was used to investigate electronic third-order nonlinearities for azo monomer 3a and conjugated azo polymer 7a. Both compounds had ultrafast nonlinearities with τ 1 ) 100 fs response time. The real part of 7a second hyperpolarizability, γ 7a , is negative, Re γ 7a ) -(4.3 ( 0.3) × 10 -46 m 5 V -2 . Polymer 7a's second hyperpolarizability also has an imaginary component, Im γ 7a ) (1.7 ( 0.3) × 10 -46 m 5 V -2 . Calculated per repeat unit, polymer |γ 7a | ) (4.6 ( 0.5) × 10 -46 m 5 V -2 and is about 780 times larger than that for monomer 3a. γ 7a was analyzed with the three-level model; γ 7a enhancement is attributed to the conjugation effects in the polymer and contributions from a two-photon state. Polymer 7a's excited state had a 3.4 ps lifetime, as determined by DFWM.
IEEE Journal of Selected Topics in Quantum Electronics, 2000
The sum-frequency spectroscopy signatures of NH-(amide A) and C = O (amide I) groups, the amide s... more The sum-frequency spectroscopy signatures of NH-(amide A) and C = O (amide I) groups, the amide segments in all proteins, are measured in thin films that consist of an ensemble of righthanded, helical poly-gamma-benzyl-L-glutamate (PBLG) macromolecules that are endgrafted and self-organized into a monomolecular film with a large degree of unidirectional order. Distinct sum-frequency spectral signatures associated with the amide A and the amide I bands are observed because of a strong noncentro-symmetry produced by intra-and intermolecular forces. Hydrogen bonding self-organizes amino and acidic groups within the molecular helical scaffold. In an endgrafted thin film, repulsive electrostatic forces between PBLG macromolecules stabilize the organization between molecules. The average orientation of the PBLG chain was measured. Imaging scans using sum-frequency generation, complemented by atomic force microscopy, were used to investigate the uniformity of orientation of the PBLG chains.
Applied Optics, 2006
An oil spill detection fluorosensing lidar for onshore or shipboard operation is described. Some ... more An oil spill detection fluorosensing lidar for onshore or shipboard operation is described. Some difficulties for its operation arise from the inclined path of rays. This is due to the increased reflection of the laser beam at the air-water interface, the decreased fluorescence signal, and the increased background light when compared with other instruments having a close-to-nadir measuring geometry. The analysis of these problems shows that they significantly reduce the detection distance in the presence of a flat water surface. However, waves on the water surface weaken the influence of the laser beam reflections but at the same time cause a variable fluorescence signal, which makes specific signal processing necessary for increased detection ranges. A fluorescence data processing method is proposed that efficiently eliminates the background water column fluorescence from signals such as yellow substance. This enables oil fluorescence to be distinguished from variable natural water fluorescence.
Applied Optics, 2006
An oil spill detection fluorosensing lidar for onshore or shipboard operation is described. Some ... more An oil spill detection fluorosensing lidar for onshore or shipboard operation is described. Some difficulties for its operation arise from the inclined path of rays. This is due to the increased reflection of the laser beam at the air-water interface, the decreased fluorescence signal, and the increased background light when compared with other instruments having a close-to-nadir measuring geometry. The analysis of these problems shows that they significantly reduce the detection distance in the presence of a flat water surface. However, waves on the water surface weaken the influence of the laser beam reflections but at the same time cause a variable fluorescence signal, which makes specific signal processing necessary for increased detection ranges. A fluorescence data processing method is proposed that efficiently eliminates the background water column fluorescence from signals such as yellow substance. This enables oil fluorescence to be distinguished from variable natural water fluorescence.
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Papers by Saulius Pakalnis