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Publications

    K. Hsien Chen, C. Lu, E. Mazur, and N. Bloembergen. 1990. “Multiplex pure rotational coherent anti-Stokes Raman spectroscopy in a molecular beam.” J. Raman Spectroscopy, 21, Pp. 819–825. Publisher's VersionAbstract
    Pure rotational coherent anti-Stokes Raman spectroscopy using a single broadband dye-laser has been applied to study rotational energy distribution of molecules in a pulsed supersonic beam. The multiplex BOXCARS configuration allows accurate determination of rotational constants and rotational temperatures, and offers a great number of advantages over other methods. The technique was applied to calibrate the cooling effect of a pulsed molecular beam of N2 and to study the rotational energy distributions of infrared multiphoton excited C2H4.
    D. S. Chung, K. Y. Christina Lee, and E. Mazur. 1990. “Spectral asymmetry in the light scattered from a nonequilibrium liquid interface.” Phys. Lett. A, 145, Pp. 348–352. Publisher's VersionAbstract
    An asymmetry of the two capillary wave peaks in the light scattered from a water surface subject to a temperature gradient parallel to the surface has been observed using a Fourier transform heterodyne technique. The sign and order of magnitude of the effect agree with linear fluctuating hydrodynamic theory.
    E. Mazur and C. Lu. 1990. “Can chemical reactions be controlled with picosecond infrared pulses?” In Nonlinear Optics and Ultrafast Phenomena, edited by R. R. Alfano and L. J. Rothberg, Pp. 7–12. Nova Publishing.Abstract
    This paper presents an overview of recently obtained results on infrared multiphoton excited molecules using coherent anti-Stokes Raman spectroscopy. The data underline the important role of collisions in the excitation process.
    E. Mazur and C. Lu. 1990. “Can chemical reactions be controlled with picosecond infrared pulses?” In Nonlinear Optics and Ultrafast Phenomena, edited by R. R. Alfano and L. J. Rothberg, Pp. 7–12. Nova Publishing.Abstract
    This paper presents an overview of recently obtained results on infrared multiphoton excited molecules using coherent anti-Stokes Raman spectroscopy. The data underline the important role of collisions in the excitation process.
    E. Mazur and C. Lu. 1990. “Nonlinear spectroscopy of infrared multiphoton excited molecules.” In Resonances, edited by P. Pershan M. Levenson, E. Mazur and Y. R. Shen, Pp. 165–174. World Scientific. Publisher's VersionAbstract
    This paper presents an overview of recently obtained results on infrared multiphoton excited molecules using coherent anti-Stokes Raman Spectroscopy. The data underline the important role of collisions in the excitation process.
    M. Buijs, J. Kai Wang, P. N. Saeta, and E. Mazur. 1990. “Laser melting of silicon: the first few picoseconds.” In Nonlinear Optics and Ultrafast Phenomena, edited by R. R. Alfano and L. J. Rothberg, Pp. 61–64. Nova Publishing.Abstract
    The use of a streak camera makes it possible to improve the study of the melting of silicon during picosecond laser annealing. Its time resolution of 1.8 ps enables us to confirm that the molten silicon is heated above the melting temperature. It also provides spatial information on the melting process.
    M. Buijs, J. Kai Wang, P. N. Saeta, and E. Mazur. 1990. “Laser melting of silicon: the first few picoseconds.” In Nonlinear Optics and Ultrafast Phenomena, edited by R. R. Alfano and L. J. Rothberg, Pp. 61–64. Nova Publishing.Abstract
    The use of a streak camera makes it possible to improve the study of the melting of silicon during picosecond laser annealing. Its time resolution of 1.8 ps enables us to confirm that the molten silicon is heated above the melting temperature. It also provides spatial information on the melting process.
    K. Hsien Chen. 1989. “Spontaneous Raman and coherent anti-Stokes Raman spectroscopy of infrared multiphoton excited molecules”. Publisher's VersionAbstract
    This thesis is a study of infrared multiphoton excitation using spontaneous and coherent anti-Stokes Raman spectroscopy. The spontaneous Raman measurements provide information on the intramolecular vibrational energy distribution over the different modes. This information is complemented by the CARS measurements which make it possible to perform state-specific studies of the vibrational and rotational distribution. For SF6, the time-resolved spontaneous Raman measurements show complete equilibration of energy from the pump mode to other vibrational modes. In contrast, for smaller molecules such as CF2Cl2, a nonthermal energy distribution is observed after excitation. These measurements therefore disprove the general belief that the intramolecular energy distribution in infrared multiphoton molecules is always in equilibrium. The CARS measurements on bulk OCS provide values for the anharmonicities and for the energy transfer rates between modes. In addition the spectra show a very fast relaxation of the vibrational energy within the n2 mode. For SO2, the CARS measurements show that it is the n1 symmetric stretching mode and not the overtone excitation of the n2 bending mode that is pumped by the CO2 laser. Moreover, it is shown that the hot bands of SO2 have been incorrectly assigned up to now. Corrected values for the anharmonicities are given. In the second half of the thesis, a pulsed supersonic molecular beam is added to the infrared multiphoton excitation study. Combined with the state- specific CARS technique, the collisionless and internally cooled molecules in the beam open the door to a more detailed study of the excitation process. Pure rotational CARS is used to study the change in rotational distribution of ethylene due to infrared excitation in the beam. The appearance of rotational holes reveal which rotational states are pumped by the CO2 laser. For OCS the evolution of the overtone population into a thermal distribution is studied, providing a value for the intramode relaxation rate. Finally, the study of SF6 in the supersonic beam sheds new light on the energy distribution in SF6 after infrared multiphoton excitation. It is shown that the two-ensemble population distribution observed by other investigators after infrared multiphoton excitation involves a considerable amount of collisional relaxation.
    K. Hsien Chen, J. Wang, and E. Mazur. 1989. “Chen, Wang, and Mazur Reply.” Phys. Rev. Lett., 63, Pp. 1534–1534. Publisher's VersionAbstract
    A reply to a Comment by A.L. Malinovsky and E.A. Ryabov on an article in the Physical Review Letters by Chen, Wang and Mazur. Their comment is attached to this reply.
    K. Hsien Chen, C. Lu, L. Anibal Avils, E. Mazur, N. Bloembergen, and M. J. Shultz. 1989. “Multiplex coherent anti-Stokes Raman spectroscopy study of infrared-multiphoton-excited OCS.” J. Chem. Phys., 91, Pp. 1462–1468. Publisher's VersionAbstract
    The vibrational energy distribution following v2 overtone excitation of OCS by a pulsed CO2 laser is studied by monitoring the coherent anti-Stokes Raman spectrum of the v1 mode. Because of the slow energy transfer from the pumped mode to other modes, and because the anharmonicity of the v2 mode is small, OCS is an ideal system for studying the interaction of an intense infrared laser field with a single, nearly harmonic, oscillator. From the spectra the cross-anharmonicities of the of the n1 mode are determined to be x12 = -6.0 cm-1 and x13 = -2.7 cm-1, respectively. The time-dependence of the spectra provides information on V-V energy transfer rates. In particular, the measurements put a lower limit of kv2->v2 = 1 ms-1 torr-1 on the vibrational relaxation rate within n2 mode. At high excitation, the temperature of the v2 mode rises up to 2000 K, and hot bands are observed up to the v = 4 level. This fourth overtone peak is split because of either a Fermi resonance or vibrational angular momentum splitting.
    K. Hsien Chen, C. Lu, E. Mazur, N. Bloembergen, and M. J. Shultz. 1989. “Multiplex CARS study of infrared-multiphoton-excited OCS.” In Laser Spectroscopy IX, edited by A. Mooradian M. Feld and J. Thomas, Pp. 232–233. Academic Press. Publisher's VersionAbstract
    {Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive and efficient technique to monitor molecular vibrational and rotational distributions. This paper reports on the application of CARS to study the dynamics of OCS after infrared multiphoton excitation. The OCS molecule has three widely separated fundamental modes ( V1 = 859 cm1
    K. Y. Christina Lee, D. S. Chung, S. Rno, and E. Mazur. 1989. “Fourier transform heterodyne spectroscopy of liquid interfaces.” In Laser Materials and Laser Spectroscopy, edited by Z. Wang and Z. Zhang, Pp. 316–319. World Scientific. Publisher's VersionAbstract
    By acousto-optically shifting the local oscillator in a heterodyne set-up, a spectral resolution of better than 150 mHz can be obtained. Applications of the technique to the study of interfacial phenomena are discussed.
    J. Kai Wang, P. N. Saeta, M. Buijs, E. Mazur, and M. Malvezzi. 1989. “Single shot reflectivity study of the picosecond melting of silicon using a streak camera.” In Ultrafast Phenomena VI, edited by C. B. Harris T. Yajima, K. Yoshihara and S. Shionoya, Pp. 236–239. Springer-Verlag.Abstract
    Numerous Investigations on the phase transition of silicon during picosecond laser annealing have been performed in recent years. it has been well established that the silicon surface melts during a picosecond laser pulse. Because liquid silicon is a metal, the reflectivity of the increases on melting. This has indeed been observed using optical pump-and-probe techniques. Standard picosecond pump-and-probe measurements, however, have some serious inherent drawbacks. First, they cannot resolve reflectivity changes that occur on a time scale of a few picoseconds, because they integrate over the duration of the probe pulse (tyically 20 ps or more). Second, they determine the time probile of the reflectivity for every pump fluence in a step-wise manner by varying the delay beteween the pump and the probe pulse. This introduces a large amount of scatter in the data points, due to shot-to-shot variations in the pump fluence, and requires a large amount of data to be taken for every time probile of the reflectivity. Also, they provide no spatial information on the melting process.To Obtain spatial resolution, a better time resolution, and to measure the time profile of the reflectivity on a single-shot basis, we use a streak camera with a time resolution of 1.8 ps for the detection of the probe pulse.
    J. Kai Wang, P. N. Saeta, M. Buijs, E. Mazur, and M. Malvezzi. 1989. “Single shot reflectivity study of the picosecond melting of silicon using a streak camera.” In Ultrafast Phenomena VI, edited by C. B. Harris T. Yajima, K. Yoshihara and S. Shionoya, Pp. 236–239. Springer-Verlag.Abstract
    Numerous Investigations on the phase transition of silicon during picosecond laser annealing have been performed in recent years. it has been well established that the silicon surface melts during a picosecond laser pulse. Because liquid silicon is a metal, the reflectivity of the increases on melting. This has indeed been observed using optical pump-and-probe techniques. Standard picosecond pump-and-probe measurements, however, have some serious inherent drawbacks. First, they cannot resolve reflectivity changes that occur on a time scale of a few picoseconds, because they integrate over the duration of the probe pulse (tyically 20 ps or more). Second, they determine the time probile of the reflectivity for every pump fluence in a step-wise manner by varying the delay beteween the pump and the probe pulse. This introduces a large amount of scatter in the data points, due to shot-to-shot variations in the pump fluence, and requires a large amount of data to be taken for every time probile of the reflectivity. Also, they provide no spatial information on the melting process.To Obtain spatial resolution, a better time resolution, and to measure the time profile of the reflectivity on a single-shot basis, we use a streak camera with a time resolution of 1.8 ps for the detection of the probe pulse.
    D. S. Chung, K. Y. Christina Lee, and E. Mazur. 1989. “Milli-Hertz Surface Spectroscopy.” In Laser Spectroscopy IX, edited by A. Mooradian M. Feld and J. Thomas, Pp. 216–219. Academic Press. Publisher's VersionAbstract
    A technique that has been repeatedly employed in high resolution light scattering experiments is that of light beating, or heterodyne, spectroscopy. By detecting the beating signal between the scattered light and a 'local oscillator' derived from the same laser source, one can obtain ultrahigh spectral resolution, independent of the random fluctuations of the light source. We reported earlier of a novel Fourier transform heterodyne spectroscopy (FTHS) technique with high resolution which is simpler and more direct than the conventional heterodyne technique; we have since improved our resolution ten-thousand fold to the 20-Hz range. We applied this technique first to study nonequilibrium phenomena at liquid-vapor interfaces. The ultrahigh resolution also enables one to observe the very small Doppler shift of a light beam reflected from a growing silicon crystal.
    E. Mazur. 1988. “Doing Physics with Computers.” In Academic Computing, February: Pp. 18–21, 46-48. Publisher's VersionAbstract
    Are the days of watching analog meters, taking notes in thick lab books and plotting data points on graph paper gone forever? Is research in the physical sciences becoming so complex that one can no longer do research without computers? A superficial survey of the current research in physics might lead one to give an affirmative answer to these questions. It is therefore interesting to note that the award of this year's Nobel Prize in physics to Alex Mller and Georg Bednorz for their work on superconductivity was hailed as a victory for relatively simple, small-scale research. No computer was needed to show that their compound of copper, oxygen, lanthanum and barium becomes superconducting. It shows that one can still make major breakthroughs with very simple means-without computers. Computers excel at performing tedious routine tasks. Physics research on the other hand seldom entails routine work. This holds true in particular in theoretical physics: indeed, no computer has yet been able to develop a new theory. Yet one cannot deny that the presence of computers in physics research and education increases every day. Physics papers on results that have been obtained with the help of computers abound, and some fields of physics would not even exist without computers. As a physicist, I will try to analyze in this article the impact of computers on physics. I will start by analyzing the current situation using examples from my own laboratory.
    J. Wang. 1988. “The intramolecular energy distribution of infrared multiphoton excited molecules”. Publisher's VersionAbstract
    Polyatomic molecules in the electronic ground state can absorb a large number of infrared photons from a resonant high power infrared laser. For sufficiently high laser power, most molecules will even reach the dissociation limit. When this phenomenon was discovered in 1973 it was hoped that infrared multiphoton excitation would lead to the realization of 'bond-selective' laser- controlled photochemistry. Despite the selectivity of infrared excitation at low energy, however, at high excitation vibrational energy is no longer confined to the pump mode because of the interaction between vibrational modes. This thesis explores the intramolecular dynamics of infrared multiphoton excited molecules. Time-resolved spontaneous and coherent anti-Stokes Raman spectroscopy was employed to measure the energy distribution among vibrational modes immediately following infrared excitation. Both cases of intramolecular equilibrium below the dissociation threshold, as well as cases of nonequilibrium close to dissociation were found. The results are consistent with nonlinear dynamics theory.
    J. Wang, K. Hsien Chen, and E. Mazur. 1988. “Raman spectroscopy of infrared multiphoton excited molecules.” Laser Chem., 8, Pp. 97–122. Publisher's VersionAbstract
    This paper presents an overview of data obtained on the intramolecular vibrational energy distribution in infrared multiphoton excited CF2HCl, CF2Cl2, SF6 and CH3CHF2. All but CF2HCl show collisionless changes in the intensity of the spontaneous Raman signals after excitation, indicating that the excitation alters the population in the Raman active modes. A comparison of the spectrally integrated intensities of the Raman signals yields information on the distribution of vibrational energy over the modes of the molecule. The results for CF2Cl2 show a nonthermal distribution of energy after the excitation.

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