84 research outputs found
Using Wave-Packet Interferometry to Monitor the External Vibrational Control of Electronic Excitation Transfer
We investigate the control of electronic energy transfer in molecular dimers
through the preparation of specific vibrational coherences prior to electronic
excitation, and its observation by nonlinear wave-packet interferometry.
Laser-driven coherent nuclear motion can affect the instantaneous resonance
between site-excited electronic states and thereby influence short-time
electronic excitation transfer (EET). We first illustrate this control
mechanism with calculations on a dimer whose constituent monomers undergo
harmonic vibrations. We then consider the use of nonlinear wave-packet
interferometry (nl-WPI) experiments to monitor the nuclear dynamics
accompanying EET in general dimer complexes following impulsive vibrational
excitation by a sub-resonant control pulse (or control pulse sequence). In
measurements of this kind, two pairs of polarized phase-related femtosecond
pulses following the control pulse generate superpositions of coherent nuclear
wave packets in optically accessible electronic states. Interference
contributions to the time- and frequency-integrated fluorescence signal due to
overlaps among the superposed wave packets provide amplitude-level information
on the nuclear and electronic dynamics. We derive the basic expression for a
control-pulse-dependent nl-WPI signal. The electronic transition moments of the
constituent monomers are assumed to have a fixed relative orientation, while
the overall orientation of the complex is distributed isotropically. We include
the limiting case of coincident arrival by pulses within each phase-related
pair in which control-influenced nl-WPI reduces to a fluorescence-detected
pump-probe difference experiment. Numerical calculations of pump-probe signals
based on these theoretical expressions are presented in the following paper
Time-, Frequency-, and Wavevector-Resolved X-Ray Diffraction from Single Molecules
Using a quantum electrodynamic framework, we calculate the off-resonant
scattering of a broad-band X-ray pulse from a sample initially prepared in an
arbitrary superposition of electronic states. The signal consists of
single-particle (incoherent) and two-particle (coherent) contributions that
carry different particle form factors that involve different material
transitions. Single-molecule experiments involving incoherent scattering are
more influenced by inelastic processes compared to bulk measurements. The
conditions under which the technique directly measures charge densities (and
can be considered as diffraction) as opposed to correlation functions of the
charge-density are specified. The results are illustrated with time- and
wavevector-resolved signals from a single amino acid molecule (cysteine)
following an impulsive excitation by a stimulated X-ray Raman process resonant
with the sulfur K-edge. Our theory and simulations can guide future
experimental studies on the structures of nano-particles and proteins
On the chromatic roots of generalized theta graphs
The generalized theta graph \Theta_{s_1,...,s_k} consists of a pair of
endvertices joined by k internally disjoint paths of lengths s_1,...,s_k \ge 1.
We prove that the roots of the chromatic polynomial $pi(\Theta_{s_1,...,s_k},z)
of a k-ary generalized theta graph all lie in the disc |z-1| \le [1 + o(1)]
k/\log k, uniformly in the path lengths s_i. Moreover, we prove that
\Theta_{2,...,2} \simeq K_{2,k} indeed has a chromatic root of modulus [1 +
o(1)] k/\log k. Finally, for k \le 8 we prove that the generalized theta graph
with a chromatic root that maximizes |z-1| is the one with all path lengths
equal to 2; we conjecture that this holds for all k.Comment: LaTex2e, 25 pages including 2 figure
GBT Discovery of Two Binary Millisecond Pulsars in the Globular Cluster M30
We report the discovery of two binary millisecond pulsars in the
core-collapsed globular cluster M30 using the Green Bank Telescope (GBT) at 20
cm. PSR J2140-2310A (M30A) is an eclipsing 11-ms pulsar in a 4-hr circular
orbit and PSR J2140-23B (M30B) is a 13-ms pulsar in an as yet undetermined but
most likely highly eccentric (e>0.5) and relativistic orbit. Timing
observations of M30A with a 20-month baseline have provided precise
determinations of the pulsar's position (within 4" of the optical centroid of
the cluster), and spin and orbital parameters, which constrain the mass of the
companion star to be m_2 >~ 0.1Msun. The position of M30A is coincident with a
possible thermal X-ray point source found in archival Chandra data which is
most likely due to emission from hot polar caps on the neutron star. In
addition, there is a faint (V_555 ~ 23.8) star visible in archival HST F555W
data that may be the companion to the pulsar. Eclipses of the pulsed radio
emission from M30A by the ionized wind from the compact companion star show a
frequency dependent duration (\propto\nu^{-\alpha} with \alpha ~ 0.4-0.5) and
delay the pulse arrival times near eclipse ingress and egress by up to 2-3 ms.
Future observations of M30 may allow both the measurement of post-Keplerian
orbital parameters from M30B and the detection of new pulsars due to the
effects of strong diffractive scintillation.Comment: 10 pages, 6 figures, Submitted to ApJ. This version includes many
recommended modifications, an improved structure, a new author, and a
completely redone optical analysi
Monitoring conical intersections in the ring opening of furan by attosecond stimulated X-ray Raman spectroscopy
Attosecond X-ray pulses are short enough to capture snapshots of molecules undergoing nonadiabatic electron and nuclear dynamics at conical intersections (CoIns). We show that a stimulated Raman probe induced by a combination of an attosecond and a femtosecond pulse has a unique temporal and spectral resolution for probing the nonadiabatic dynamics and detecting the ultrafast (similar to 4.5 fs) passage through a CoIn. This is demonstrated by a multiconfigurational self-consistent-field study of the dynamics and spectroscopy of the furan ring-opening reaction. Trajectories generated by surface hopping simulations were used to predict Attosecond Stimulated X-ray Raman Spectroscopy signals at reactant and product structures as well as representative snapshots along the conical intersection seam. The signals are highly sensitive to the changes in nonadiabatically coupled electronic structure and geometry. (C) 2015 Author(s)
Two-Dimensional Stimulated Resonance Raman Spectroscopy of Molecules with Broadband X-ray Pulses
Expressions for the two-dimensional Stimulated x-ray Raman Spectroscopy
(2D-SXRS) signal obtained using attosecond x-ray pulses are derived. The 1D-
and 2D-SXRS signals are calculated for trans-N-methyl acetamide (NMA) with
broad bandwidth (FWHM ~14.2eV, 181 as) pulses tuned to the oxygen and nitrogen
K-edges. Crosspeaks in 2D signals reveal electronic Franck-Condon overlaps
between valence orbitals and relaxed orbitals in the presence of the core hole
Understanding excitation energy transfer in metalloporphyrin heterodimers with different linkers, bonding structures, and geometries through stimulated X-ray Raman spectroscopy
We present simulations of stimulated X-ray Raman (SXRS) signals from covalent porphyrin heterodimers with different linkers, chemical bonding structures and geometries. The signals are interpreted in terms of valence electron wavepacket motion. One- and two-color SXRS signals can jointly indicate excitation energy transfer (EET) between the porphyrin monomers. It is shown that the SXRS signals provide a novel window into EET dynamics in multiporphyrin systems, and can be used as a powerful tool to monitor the subtle chemical environment which affects EET
Eight New Millisecond Pulsars in NGC 6440 and NGC 6441
Motivated by the recent discovery of 30 new millisecond pulsars in Terzan 5,
made using the Green Bank Telescope's S-band receiver and the Pulsar Spigot
spectrometer, we have set out to use the same observing system in a systematic
search for pulsars in other globular clusters. Here we report on the discovery
of five new pulsars in NGC 6440 and three in NGC 6441; each cluster previously
had one known pulsar. Using the most recent distance estimates to these
clusters, we conclude that there are as many potentially observable pulsars in
NGC 6440 and NGC 6441 as in Terzan 5. We present timing solutions for all of
the pulsars in these globular clusters. Four of the new discoveries are in
binary systems; one of them, PSR J1748-2021B (NGC 6440B), has a wide (P_b =
20.5 d) and eccentric (e = 0.57) orbit. This allowed a measurement of its rate
of advance of periastron: 0.00391(18) degrees per year. If due to the effects
of general relativity, the total mass of this binary system is 2.92 +/- 0.20
solar masses (1 sigma), implying a median pulsar mass of 2.74 +/- 0.21 solar
masses. There is a 1 % probability that the inclination is low enough that
pulsar mass is below 2 solar masses, and 0.10 % probability that it is between
1.20 and 1.44 solar masses. If confirmed, this anomalously large mass would
strongly constrain the equation of state for dense matter. The other highly
eccentric binary, PSR J1750-37A, has e = 0.71, and periastron advance of
0.0055(3) degrees per year, implying a total system mass of 1.97 +/-0.15 solar
masses and, along with the mass function, maximum and median pulsar masses of
1.65 and 1.26 solar masses respectively.Comment: Accepted for publication by the Astrophysical Journal. 14 pages in
emulate format, 6 tables, 7 figure
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