A novel spectroscopic tool is presented for measuring the incoherent resonance decay (IRD) of selectively (6 MHz) prepared electronic states. The method utilizes electro-optic switching of a single laser mode that is on or off resonance with respect to the homogeneous molecular packets in the excited ensemble. The technique is applied to a variety of systems: gases at low and high pressures, molecular beams, and solids at low temperatures. The coherent transient of solids observed in the forward direction of the laser gives the phase memory (optical T2 processes) while the IRD measures directly the radiative and radiationless decay (optical T1 processes), and the optical transition moment between the ground state and the prepared electronic state. The theory of coherent and incoherent states is also given, and related to the different aspects of time-resolved spectroscopy of molecular beams and solids.
A novel spectroscopic tool is presented for measuring the incoherent resonance decay (IRD) of selectively (6 MHz) prepared electronic states. The method utilizes electro-optic switching of a single laser mode that is on or off resonance with respect to the homogeneous molecular packets in the excited ensemble. The technique is applied to a variety of systems: gases at low and high pressures, molecular beams, and solids at low temperatures. The coherent transient of solids observed in the forward direction of the laser gives the phase memory (optical T2 processes) while the IRD measures directly the radiative and radiationless decay (optical T1 processes), and the optical transition moment between the ground state and the prepared electronic state. The theory of coherent and incoherent states is also given, and related to the different aspects of time-resolved spectroscopy of molecular beams and solids.
A new technique is presented for measuring the incoherent resonance decay (IRD) and the coherent optical ringing of selectively (5 MHz) prepared electronic states. The method utilizes electro-optic switching of a single laser mode that is on or off resonance with respect to the homogeneous molecular packets in the excited ensemble. The technique was applied to a variety of systems (gases at low pressures, gases at relatively high pressures, and solids at low temperatures) to give information about their phase memory (optical T2 processes), radiative and radiationless decay (optical T1 processes), and to measure directly their optical transition moment between the ground state and the prepared electronic state. The theory of coherent and incoherent states is also given.
I. A new technique utilizing the electro-optic frequency switching of a single mode ( 6 Mhz) dye laser was used to observe free induction decay and optical nutation in iodine gas and optical nutation in an organic mixed crystal (pentacene in p-terphenyl) at low temperature. A new technique, incoherent resonance decay (IRO) was also developed. These experiments allow one to determine T_1 and T_2, the longitudinal and transverse relaxation times of the excited ensemble, in analogy with magnetic resonance. For I_2(g) at O∙C; T_l = 2.3 x 10^(-7) sec, T_2 ≃ 1 x 10^(-7) sec. These techniques are limited to a time scale > 10 nsec.
II. The construction and characterization of cw, mode-locked dye laser capable of putting out a continuous train of reproducible sub-picosecond pulses is described. Experiments utilizing this laser for the measurement of picosecond coherent transients and time resolved absorption spectroscopy are proposed.
Recently we have reported[1] on the measurements of electronic dephasing (T_2) and radiationless decay (T_1) in molecules using emission spectroscopy as a probe for the detection of the photon echo. This right-angle photon echo utilizes a single mode of a narrow-band (~MHz) cw dye laser and an intracavity electro-optical (EO) element that modulates the frequency of the laser.[2] Applying voltage pulses into the EO crystal switches the laser frequency from ω_0 (the peak frequency of the transition resonance) into another frequency ω. This switching procedure introduced by Brewer et al. for the observation of transients in the forward direction of the laser allows one to observe coherent transients in gases,[1,2] solids[3-5] and molecular beams.[6] Typically, the dispersion of the nonlinear EO crystal (e.g., AD*P) gives a 60 MHz frequency shift within the inhomogeneous resonance of the optical transition (width ~GHz or larger). Therefore, to switch the laser frequency further, one must use high-voltage signals that are usually hard to obtain especially for multiple-pulse experiments. Secondly, there is a limit on the frequency shift which is conditioned by the mode spacing of the cavity. Thirdly, because the laser is switched into a new group of molecules some complications may arise in interpreting the transient spectra especially in large molecules. This is because in large molecules the density of excited states is high and thus the buildup of transients of w molecules (which could be complex) will be superimposed on the decay of ω_0 molecules. Finally, off-resonance effects (e.g., beats) between the molecules at frequency ω and ω_0 cannot be avoided.
Traditionally, the phenomenon of optical coherence has been observed by detecting coherent transients (i.e., optical nutation, FID, photon echoes) propagating along the laser beam following passage through an absorbing medium. Recently, we have demonstrated that detection of the incoherent (spontaneous) emission from selectively prepared states provides the optical coherence of the ensemble. Our observation of the nutation and the photon echo at right angles to the exciting beam gives directly optical T_1, T_2, and the moment of the laser excited transition. This technique is simple and not susceptible to some of the problems inherent in coherent absorption experiments. Details about this laser technique will be presented. We will also discuss the different dynamical information one obtains about excited states in gases, molecular beams and solids.
A simple method for observing coherent optical-transient phenomena is described. Effects such as photon echoes and free induction decay (FID) should allow studies of the dynamic properties of atoms, molecules, and solids on a time scale ranging from milliseconds to \ensuremath{\sim} 50 ps. The frequency of a cw dye laser is switched with an intracavity electro-optic modulator, producing in an external sample coherent transients which are detected in the forward beam. Measurements of long dephasing times are possible, as in FID, because the laser can oscillate indefinitely at the new frequency following a step-function switching pulse. Measurements of short decay times are also practical because the switching time is not restricted by the laser gain or cavity ringing time. In addition, the dye's dephasing time of a few picoseconds or less is too rapid to interfere with these observations. Since the pulse sequence can be preselected, the entire class of coherent optical transients is accessible. The method thus incorporates all of the advantages inherent in Stark switching, including heterodyne detection and high sensitivity, without being restricted to Stark-tunable systems. Laser frequency switching is discussed also in terms of a theoretical model which exposes the time-dependent properties of a phase-modulated cavity mode. Dephasing studies have been carried out thus far in atomic and molecular gases and low-temperature solids containing inorganic impurity ions or large organic molecules. The utility of the FID effect is emphasized where precise dephasing times are obtained from time-frequency Fourier transformation of FID signals using a digital computer. The first quantitative and detailed test of FID theory is provided by these experimental techniques.
This paper outlines new ways for separating the different channels of optical dephasing of molecules in beams. It is shown that both the population loss and the optical phase relaxation rates can be obtained under collisionless conditions (molecular beams). These dephasing rates, which measure directly the homogeneous width of the prepared resonance, can be obtained with an energy resolution of better than one part in 108. Under these conditions the solution of the density matrix equations of motion is given for effusive and nozzle beams which represent a statistically open ensemble. The results, which we applied to our recent measurements of optical T1 and T2, are discussed in different limits of power broadening, beam geometry, detector characteristics, temperature of the oven (or material container), and the transit time the molecules of different velocities spend in the laser beam. Furthermore, we indicate that the treatment of coherent transients in beams utilizing conventional Bloch equations is not valid since there is a loss of optically excited molecules. Finally, we discuss possible differences between small and large molecules when they undergo radiationless (or reactive) processes following the selective laser excitation (∼10 kHz–10 MHz).