Non-adiabatic dissociation dynamics of Ar⋯I2 (E, v) intermolecular vibrational levels probed using velocity-map imaging.

Ion time-of-flight velocity-map imaging was used to measure the kinetic-energy distributions of the I2 ion-pair fragments formed after photoexcitation of Ar⋯I2 complexes to intermolecular vibrational levels bound within the Ar + I2 (E, vE = 0-2) potential energy surfaces. The kinetic-energy distributions of the I2 products indicate that complexes in the Ar⋯I2 (E, vE) levels preferentially dissociate into I2 in the D and ß ion-pair states with no change in I2 vibrational excitation. The energetics of the levels prepared suggest that there is a non-adiabatic coupling of the initially prepared levels with the continuum of states lying above the Ar + I2 (D, vD = vE) and Ar + I2 (ß, vß = vE) dissociation limits. The angular anisotropies of the I2 product signals collected for many of the Ar⋯I2 (E, vE) levels have maxima parallel to the laser polarization axis. This contradicts expectations for the prompt dissociation of complexes with T-shaped geometries, which would result in images with maxima perpendicular to the polarization axis. These anisotropies suggest that there is a perturbation of the transition moment in these clusters or there are additional intermolecular interactions, likely those sampled while traversing above the attractive wells of the lower-energy potentials during dissociation. I2 (D', vD') products are also identified when preparing several of the low-lying levels localized in the T-shaped well of the Ar + I2 (E, vE = 0-2) potentials, and they are formed in multiple νD' vibrational levels spanning energy ranges up to 500 cm-1.

Piezo-deformable mirrors for active mode matching in advanced LIGO.

The detectors of the laser interferometer gravitational-wave observatory (LIGO) are broadly limited by the quantum noise and rely on the injection of squeezed states of light to achieve their full sensitivity. Squeezing improvement is limited by mode mismatch between the elements of the squeezer and the interferometer. In the current LIGO detectors, there is no way to actively mitigate this mode mismatch. This paper presents a new deformable mirror for wavefront control that meets the active mode matching requirements of advanced LIGO. The active element is a piezo-electric transducer, which actuates on the radius of curvature of a 5 mm thick mirror via an axisymmetric flexure. The operating range of the deformable mirror is 120±8 mD in vacuum and an additional 200 mD adjustment range accessible out of vacuum. Combining the operating range and the adjustable static offset, it is possible to deform a flat mirror from -65 mD to -385 mD. The measured bandwidth of the actuator and driver electronics is 6.8 Hz. The scattering into higher-order modes is measured to be <0.2% over the nominal beam radius. These piezo-deformable mirrors meet the stringent noise and vacuum requirements of advanced LIGO and will be used for the next observing run (O4) to control the mode-matching between the squeezer and the interferometer.

Vibrational predissociation versus intramolecular vibrational energy redistribution (IVR) in rare gas⋯dihalogen complexes: IVR identified in Ar⋯I2(B, ν') using velocity-map imaging.

The competition between multiple pathways sampled during the energetic relaxation of excited molecules can be difficult to experimentally decipher. The rare gas···dihalogen van der Waals complexes have remained key systems for exploring the competition between relaxation pathways, such as intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP). As these mechanisms can yield the same products, the relaxation pathways traversed are often deduced from the excitation spectra or product-state distributions. In addition to a brief perspective on IVR and VP in rare gas⋯dihalogen complexes, we present new results obtained using time-of-flight velocity-map imaging (VMI) on T-shaped Ar⋯I2(B, ν', n' = 0) complexes that illustrate how contributions from these two pathways can be separated. The angular anisotropies of the ion images collected for the I2(B, ν < ν') fragments indicate the products for certain Ar⋯I2(B, ν', n' = 0) levels are weighted along the direction perpendicular to the laser-polarization axis. These distributions are consistent with prompt dissociation of the T-shaped excited-state complexes, likely via direct VP. The distributions measured for other Ar⋯I2(B, ν', n' = 0) levels are preferentially along the laser-polarization axis. These initially prepared levels must undergo IVR with nearly resonant Ar⋯I2(B, ν < ν', n > 0) intermolecular vibrational levels that sample linear Ar-I-I orientations during dissociation.

Electronic predissociation in rare gas-dihalogen complexes.

The role of electronic predissociation (EP) in the dissociation dynamics of rare gas⋯dihalogen complexes (Rg⋯X2) prepared in the B electronic state was probed using ion time-of-flight velocity-map imaging. Specifically, EP of complexes prepared in the T-shaped Ar⋯I2, Ne⋯I2, He⋯I2, Ar⋯Br2, Ne⋯Br2, and He⋯Br2 levels with varying amounts of X2 vibrational excitation, ν', was investigated. The atomic I(2P3/2) or Br(2P3/2) EP fragments were probed using ion time-of-flight velocity-map imaging. Definitive evidence for EP was observed only for the Ar⋯I2 complex, and it occurs for all of the T-shaped intermolecular levels investigated, those with ν' = 12-22, 24, and 25. The relative yields for EP in these levels measured as a function of ν' are consistent with previously reported yields for the competing mechanism of vibrational predissociation. The anisotropies of the I+ images collected for Ar⋯I2 indicate that EP is occurring on timescales shorter than the rotational periods of the complex. The kinetic energy distributions of the departing I-atom fragments suggest that EP occurs from an asymmetric geometry rather than the rigid T-shaped geometry for many of the Ar⋯I2 levels prepared. These findings indicate that intramolecular vibrational redistribution of these initially prepared T-shaped levels to excited levels bound within a lower-energy intermolecular potential occurs prior to EP.