Interrogation of the Dissociation Dynamics of Ar∙∙∙I2

In this work, ion time-of-flight velocity-map imaging (TOF-VMI) is combined with laser-induced fluorescence (LIF) to accurately measure the ground-state binding energies of the T-shaped and linear Ar∙∙∙I2(X,νʺ=0) complexes. LIF spectra were used to identify the transitions used to preferentially excite either the T-shaped or linear Ar∙∙∙I2(B,νʹ) conformer. The kinetic energy distributions of I2(B,ν=νʹ−3) fragments formed with low kinetic energies via dissociation of the initially prepared Ar∙∙∙I2(B,νʹ) intermolecular levels were imaged to measure the binding energies of the T-shaped and linear conformers. The linear conformer is energetically preferred with a binding energy of 250.2(2.7) cm−1, over the T-shaped conformer, which has a binding energy of 240.5(3.6) cm−1. Most of the experimental results in this dissertation characterize the dynamics of the Ar + I2(B,νʹ) potential energy surface (PES). The intermolecular vibrational levels bound in the Ar + I2(B,νʹ=20-23) PESs are recorded using two-color action spectroscopy. Transitions of both the T-shaped and linear ground-state conformers are identified. There is a high density of intermolecular features observed in the spectra, due in part to the depth of the Ar + I2(B,νʹ) excited-state PESs and the resultant overlap of features bound within the different PES. Comparisons of the spectral shifts of the features from different I2 B−X. νʹ−0 bands are used to identify the levels bound within each of the Ar + I2(B,νʹ) PESs. At least 17 bound intermolecular levels are identified. The lowest levels correspond to the Ar atom localized in the T-shaped well, and the highest levels correspond to the Ar atom delocalized about the I2 molecule. Identification of these levels and their associated geometries are used to characterize the nature of intramolecular vibrational redistribution (IVR) upon excitation of the complex. Ion TOF-VMI is used to characterize the dissociation dynamics of T-shaped Ar∙∙∙I2(B,νʹ). The angular anisotropy of the I2(B,ν\u3cνʹ) fragments are measured to probe the energetics and geometries of the Ar∙∙∙I2(B,νʹ) intermolecular vibrational levels sampled during dissociation. The anisotropy is compared with the probability distributions of the levels accessible in order to identify the Ar∙∙∙I2(B,ν\u3cνʹ) levels that are likely involved in the IVR mechanism. For certain νʹ, the I2 angular distributions indicate IVR dominates over direct vibrational predissociation (VP). IVR is also involved in electronic predissociation (EP). VMI is used to detect atomic I(2P3/2) fragments formed from EP of the prepared T-shaped Ar∙∙∙I2(B,νʹ) level. The angular distributions of the I(2P3/2) fragments confirm that EP occurs via nonadiabatic interactions with dissociative electronic states. The kinetic energy distributions of the departing fragments have a bimodal distribution, which indicates the EP occurs from an asymmetric geometry, not the rigid T-shaped geometry of the initially prepared Ar∙∙∙I2(B,νʹ) level. IVR occurs prior to interaction with the repulsive electronic state and the changes in geometry associated with IVR support theoretical predictions that the a′(0g+) dissociative state is the one that most contributes to EP. Comparison of the I(2P3/2) yield from EP and the I2(B,ν\u3cν′) yield from VP reveals that the two processes are in competition with each other. The Ar + I2(E,ν†) interactions were also investigated. Laser-induced fluorescence spectra identifying a large number of Ar···I2(E,ν†=0-3) intermolecular vibrational levels were recorded using metastable Ar···I2(B,ν=23) levels as intermediates in a two-color, two-photon excitation scheme. A binding energy of 410.3(3.6) cm–1 is established for the T-shaped complex in the Ar + I2(E,ν†=0) potential energy surface

Search for intermediate-mass black hole binaries in the third observing run of Advanced LIGO and Advanced Virgo

International audienceIntermediate-mass black holes (IMBHs) span the approximate mass range 100−105 M⊙, between black holes (BHs) that formed by stellar collapse and the supermassive BHs at the centers of galaxies. Mergers of IMBH binaries are the most energetic gravitational-wave sources accessible by the terrestrial detector network. Searches of the first two observing runs of Advanced LIGO and Advanced Virgo did not yield any significant IMBH binary signals. In the third observing run (O3), the increased network sensitivity enabled the detection of GW190521, a signal consistent with a binary merger of mass ∼150 M⊙ providing direct evidence of IMBH formation. Here, we report on a dedicated search of O3 data for further IMBH binary mergers, combining both modeled (matched filter) and model-independent search methods. We find some marginal candidates, but none are sufficiently significant to indicate detection of further IMBH mergers. We quantify the sensitivity of the individual search methods and of the combined search using a suite of IMBH binary signals obtained via numerical relativity, including the effects of spins misaligned with the binary orbital axis, and present the resulting upper limits on astrophysical merger rates. Our most stringent limit is for equal mass and aligned spin BH binary of total mass 200 M⊙ and effective aligned spin 0.8 at 0.056 Gpc−3 yr−1 (90% confidence), a factor of 3.5 more constraining than previous LIGO-Virgo limits. We also update the estimated rate of mergers similar to GW190521 to 0.08 Gpc−3 yr−1.Key words: gravitational waves / stars: black holes / black hole physicsCorresponding author: W. Del Pozzo, e-mail: [email protected]† Deceased, August 2020