ALBERT

Notes: Based on optical fundamentals, we present in this article a practical method to obtain an interference fringe-free transmission spectrum for hydrogenated amorphous solid thin films. From this spectrum, reliable optical properties, such as the Urbach edge and optical band gap of the thin films, can be extrapolated directly. In terms of the Brewster angle accuracy, the margins of error of the proposed method due to material dispersion are less than ±1% for hydrogenated amorphous silicon and less than ±1.2% for hydrogenated amorphous silicon nitride. These figures are less than the detectable limit of the proposed method. © 2000 American Institute of Physics.

Notes: In this letter, we show that normal-incidence spectroscopic ellipsometry can be used for high-accuracy topography measurements on surface relief gratings. We present both experimental and theoretical results which show that spectroscopic ellipsometry or reflectance-difference spectroscopy at near-normal incidence coupled with vector diffraction theory for data analysis is capable of high-accuracy critical dimension (CD), feature height, and sidewall angle measurements in the extreme submicron regime. Quantitative comparisons of optical and cross-sectional scanning electron microscopy (SEM) topography measurements from a number of 350 nm line/space reactive-ion-etched Si gratings demonstrate the strong potential for in situ etching monitoring. This technique can be used for both ex situ and in situ applications and has the potential to replace the use of CD-SEM measurements in some applications. © 2001 American Institute of Physics.

Notes: Using a mass analyzed threshold ionization (MATI) spectrometer, the spectra of the monomer, dimer, and trimer of sodium were measured. Intense dimer signals both at the ionization threshold and at Rydberg resonances of the monomer were observed. Below the adiabatic threshold of the dimer, a trimer spectrum which showed vibrational progressions similar to those of the dimer was also recorded. In both cases, the high energy edge of the heavier cluster was shifted to a lower energy compared with that of the lighter cluster. The monomer time-of-flight spectrum at the Rydberg resonances was diffuse, while the time-of-flight spectrum of the dimer was sharp and intense. Based on the continuity of oscillator strength and further energetic analysis, we conclude that the dimer signal at the Rydberg resonances of the monomer originated from collisional association between a Rydberg state atom and a ground state atom. Similarly, the trimer signal was from the combination between a Rydberg state dimer and a ground state atom. The combined product from collision was in a Rydberg state that can be field ionized during MATI detection. This work offers direct experimental evidence for the formation of high Rydberg state species through collisional recombination. © 2000 American Institute of Physics.

Notes: The direction of the transition dipole moment of nitrobenzene between 230 and 250 nm was determined by orienting gas-phase molecules in a strong, uniform electric field. Oriented nitrobenzene was photodissociated with linearly polarized light, and the NO fragments were detected by resonantly enhanced multiphoton ionization (REMPI). When the polarization direction of the photolysis laser was perpendicular (rather than parallel) to the orientation field, a 44% enhancement in the NO signal was observed. This implies a predominantly perpendicular relationship between the transition dipole and the permanent dipole. However, the experimentally observed enhancement falls below that expected of a pure perpendicular transition, indicating the presence of a second potential-energy surface that is simultaneously accessed through a parallel transition. Quantitative analysis indicates that the parallel transition contributes 20% of the overall oscillator strength. © 2000 American Institute of Physics.

Notes: Orientation of pyrimidine in a strong electric field was measured using resonantly enhanced multiphoton ionization (REMPI) and laser induced fluorescence (LIF). The ion and fluorescence yields showed preference for perpendicular excitation relative to the orientation field, implying a perpendicular relationship between the permanent dipole and the transition dipole. Calculation results using a linear variation method reproduced the observed spectral features, overall transition intensity, and polarization preference of the excitation laser. The permanent dipole of the S1 state of pyrimidine was thereby determined to be +0.6 Debye. Measurements of polarization preferences in photoexcitation, i.e., linear dichroism, provide a direct approach for determination of transition dipole moments. A general theory of deriving directions of transition dipoles relative to permanent dipoles based on this type of measurement/calculation was also developed. In addition, we report observations of complex relaxation dynamics of pyrimidine in an electric field. At 50 kV/cm, the overall fluorescence yield was quenched to a quarter of its value under field free conditions. The spectral intensity distribution exhibited dependence on the delay time of the ionization laser in the REMPI experiment. Qualitative assignments of the REMPI spectra revealed that the loss of signal strength with delay time was primarily from levels containing high rotational angular momenta. Elimination of contributions from levels with M′≥3 in the calculation was sufficient to reproduce experimental spectra recorded with a delay time of 200 ns. These observations and interpretations agree with previous reports on photophysical properties of pyrimidine, including relaxation and quenching in a magnetic field. © 1999 American Institute of Physics.

Notes: Degenerate four wave mixing (DFWM) in supersonically cooled pyrazine was investigated using different polarization combinations of the three input beams. A common feature of these spectra was the strong Q branch, stronger than the corresponding spectra for a single photon process. The ratios for all the rotational branches demonstrated strong dependence on the polarization combination. The YYXX combination showed similar intensities for the P and R branches, while the YXYX combination had a much stronger R branch. The Q branch, relative to the P branch, was the strongest for the YYYY combination. All experiments probed for the same electronic transition with the same Hönl–London factor; therefore the variation in the rotational branching ratios was solely an effect of the polarization combinations. This polarization dependence of DFWM can be traced back to the selectivity in the magnetic quantum number for this multiphoton process. Based on the theoretical framework by Williams et al. [J. Chem. Phys. 101, 1072 (1994)], these spectra were simulated successfully. The calculations and analysis of the experimental conditions revealed that among the twelve gratings included in the original theory, only three of them had observable contributions to the experimental spectra, i.e., the gratings formed through the ground-state molecules via the two forward beams. The other nine gratings either were washed out or decayed in the collision free environment (supersonic expansion) under an excitation pulse of 7 ns. This argument was further confirmed by another experiment using a time delay between the backward probe beam and the two forward beams. The spectra recorded with a 15 ns delay in the probe beam (longer than the pulse duration of each input beam) were similar to those without delays. These results implied that (1) the backward beam did not participate in the grating formation process even when it arrived at the interaction region simultaneously with the two forward beams. (2) Relaxation from any of the three contributing gratings was not observable. (3) The fast intersystem crossing, known to exist in pyrazine, had no effect on the grating formation and relaxation processes. Although complex in nature, degenerate four wave mixing offers versatility in experimental arrangements, providing both dynamics and spectroscopy information. © 1997 American Institute of Physics.