Modelling submillimetre spectra of the protostellar infall candidates NGC1333-IRAS2 and Serpens SMM4

We present a radiative transfer model, which is applicable to the study of submillimetre spectral line observations of protostellar envelopes. The model uses an exact, non-LTE, spherically symmetric radiative transfer `Stenholm' method, which numerically solves the radiative transfer problem by the process of `Lambda-iteration'. We also present submillimetre spectral line data of the Class 0 protostars NGC1333-IRAS2 and Serpens SMM4. We examine the physical constraints which can be used to limit the number and range of parameters used in protostellar envelope models, and identify the turbulent velocity and tracer molecule abundance as the principle sources of uncertainty in the radiative transfer modelling. We explore the trends in the appearance of the predicted line profiles as key parameters in the models are varied. We find that the separation of the two peaks of a typical infall profile is dependent not on the evolutionary status of the collapsing protostar, but on the turbulent velocity dispersion in the envelope. We also find that the line shapes can be significantly altered by rotation. Fits are found for the observed line profiles of IRAS2 and SMM4 using plausible infall model parameters. The density and velocity profiles in our best fit models are inconsistent with a singular isothermal sphere model. We find better agreement with a form of collapse which assumes non-static initial conditions. We also find some evidence that the infall velocities are retarded from free-fall towards the centre of the cloud, probably by rotation, and that the envelope of SMM4 is rotationally flattened.Comment: Accepted by MNRA

Simulations of time-dependent fluorescence in nano-confined solvents

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/120/17/10.1063/1.1691391The time-dependent fluorescence of a model diatomic molecule with a charge-transfer electronic transition in confined solvents has been simulated. The effect of confining the solvent is examined by comparing results for solutions contained within hydrophobic spherical cavities of varying size (radii of 10–20 Å). In previous work [J. Chem. Phys. 118, 6618 (2002)] it was found that the solute position in the cavity critically affects the absorption and fluorescence spectra and their dependence on cavity size. Here we examine the effect of cavity size on the time-dependent fluorescence, a common experimental probe of solvent dynamics. The present results confirm a prediction that motion of the solute in the cavity after excitation can be important in the time-dependent fluorescence. The effects of solvent density are also considered. The results are discussed in the context of interpreting time-dependent fluorescencemeasurements of confined solvent systems

New methods for quantum mechanical reaction dynamics

Quantum mechanical methods are developed to describe the dynamics of bimolecular chemical reactions. We focus on developing approaches for directly calculating the desired quantity of interest. Methods for the calculation of single matrix elements of the scattering matrix (S-matrix) and initial state-selected reaction probabilities are presented. This is accomplished by the use of absorbing boundary conditions (ABC) to obtain a localized (L{sup 2}) representation of the outgoing wave scattering Green`s function. This approach enables the efficient calculation of only a single column of the S-matrix with a proportionate savings in effort over the calculation of the entire S-matrix. Applying this method to the calculation of the initial (or final) state-selected reaction probability, a more averaged quantity, requires even less effort than the state-to-state S-matrix elements. It is shown how the same representation of the Green`s function can be effectively applied to the calculation of negative ion photodetachment intensities. Photodetachment spectroscopy of the anion ABC{sup -} can be a very useful method for obtaining detailed information about the neutral ABC potential energy surface, particularly if the ABC{sup -} geometry is similar to the transition state of the neutral ABC. Total and arrangement-selected photodetachment spectra are calculated for the H{sub 3}O{sup -} system, providing information about the potential energy surface for the OH + H{sub 2} reaction when compared with experimental results. Finally, we present methods for the direct calculation of the thermal rate constant from the flux-position and flux-flux correlation functions. The spirit of transition state theory is invoked by concentrating on the short time dynamics in the area around the transition state that determine reactivity. These methods are made efficient by evaluating the required quantum mechanical trace in the basis of eigenstates of the Boltzmannized flux operator

A general method for implementing vibrationally adiabatic mixed quantum-classical simulations

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/118/3/10.1063/1.1528891.An approach for carrying out vibrationally adiabatic mixed quantum-classical molecular dynamics simulations is presented. An appropriate integration scheme is described for the vibrationally adiabatic equations of motion of a diatomic solute in a monatomic solvent and an approach for calculating the adiabatic energy levels is presented. Specifically, an iterative Lanczos algorithm with full reorthogonalization is used to solve for the lowest few vibrational eigenvalues and eigenfunctions. The eigenfunctions at one time step in a mixed quantum-classical trajectory are used to initiate the Lanczos calculation at the next time step. The basis set size is reduced by using a potential-optimized discrete variable representation. As a demonstration the problem of a homonuclear diatomic molecule in a rare gas fluid (N2 in Ar) has been treated. The approach is shown to be efficient and accurate. An important advantage of this approach is that it can be straightforwardly applied to polyatomic solutes that have multiple vibrational degrees-of-freedom that must be quantized

A Monte Carlo study of spectroscopy in nanoconfined solvents

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/117/14/10.1063/1.1505436The absorption and fluorescence spectra of a model diatomic molecule with a charge-transfer electronic transition are simulated. The effect of confining the solvent in which the diatomic molecule is dissolved is examined by comparing results for solutions contained within hydrophobic spherical cavities of varying size (radii of 10–20 Å). The effect of solvent polarity is also considered by comparing results of simulations with CH3I and CH3CNsolvents. The spectra, solute radial and angular distribution functions, and free energy surfaces in the solvent and radial solute position coordinates are presented and discussed. It is found that the solute position in the cavity critically affects the absorption and fluorescence spectra and their dependence on cavity size. The implications of these results for time-dependent fluorescence measurements are discussed

Empirical Support for the PCAOB’s Elimination of the Independent Auditor’s Opinion Regarding Management’s Assessment of Internal Control

In an attempt to bolster public confidence in the accounting profession, the PCAOB issued several standards that were intended to address weaknesses in audit reporting and to increase public confidence in financial reporting. One of these standards, Auditing Standard No.2, added two opinions on an enterprise’s internal control to audit reporting requirements. This Standard was superseded by Auditing Standard No. 5, which eliminated one of these opinions. The purpose of this paper is to examine the efficacy of the elimination of the auditor’s opinion regarding management’s assessment of internal control. The data in this study were taken from 10-K reports filed by Fortune 500 Companies in 2004-2007. From the 10-K reports, copies of audit reports were gathered for 114 of the 120 largest companies and the opinions (unqualified, qualified, or adverse) were recorded

On the ‘‘direct’’ calculation of thermal rate constants

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/102/19/10.1063/1.469053.We present a new approach for the direct (and correct) calculation of thermal rate constantsk(T) (‘‘direct’’ meaning that one avoids having to solve the state‐to‐state reactive scattering problem, and ‘‘correct’’ meaning that the method contains no inherent approximations). The rate constant is obtained from the long time limit of the flux‐position correlation function,C f,s (t), whose calculation is made efficient by taking advantage of the low rank of the flux operator. Specifically, the trace required to obtain C f,s (t) is evaluated by a Lanczos iteration procedure which calculates only the nonzero eigenvalues. The propagation in complex time, t c =t−iℏβ/2, is carried out using a Chebychev expansion. This method is seen to be both accurate and efficient by application to the Eckart barrier, the collinear H+H2reaction, and the three‐dimensional D+H2 (J=0) reaction

On the “direct” calculation of thermal rate constants. II. The flux-flux autocorrelation function with absorbing potentials, with application to the O+HCl→OH+Cl reaction

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/106/1/10.1063/1.474109We present a method for obtaining the thermal rate constant directly (i.e., without first solving the state-to-state reactive scattering problem) from the time integral of the flux-flux autocorrelation function, Cff(t). The quantum mechanical trace involved in calculating Cff(t) is efficiently evaluated by taking advantage of the low rank of the Boltzmannized flux operator. The time propagation is carried out with a Hamiltonian which includes imaginary absorbing potentials in the reactant and product exit channels. These potentials eliminate reflection from the edge of the finite basis and ensure that Cff(t) goes to zero at long times. In addition, the basis can then be contracted to represent a smaller area around the interaction region. We present results of this method applied to the O+HCl reaction using the J-shifting and helicity conserving approximations to include nonzero total angular momentum. The calculated rate constants are compared to experimental and previous theoretical results. Finally, the effect of deuteration (the O+DCl reaction) on the rate constant is examined

Quantum mechanical transition state theory and tunneling corrections

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/110/9/10.1063/1.478304An efficient implementation of the quantum mechanical transition state theory recently proposed by Hansen and Andersen [J. Chem. Phys. 101, 6032 (1994); J. Phys. Chem. 100, 1137 (1996)] is presented. Their method approximates the flux–flux autocorrelation function by using short-time information to fit an assumed functional form (with physically correct properties). The approach described here exploits the low rank of the half-Boltzmannized flux operator, thereby facilitating application to reactions involving many degrees of freedom. In addition, we show how the quantum transition state theory can be used to obtain tunneling corrections within the framework of more traditional transition state theory approaches, i.e., those making an assumption of separability. Directions for possible improvements of the theory are discussed

W49A North - Global or Local or No Collapse?

We attempt to fit observations with 5" resolution of the J=2-1 transition of CS in the directions of H II regions A, B, and G of W49A North as well as observations with 20" resolution of the J=2-1, 3-2, 5-4, and 7-6 transitions in the directions of H II regions A and G by using radiative transfer calculations. These calculations predict the intensity profiles resulting from several spherical clouds along the line of sight. We consider three models: global collapse of a very large (5 pc radius) cloud, localized collapse from smaller (1 pc) clouds around individual H II regions, and multiple, static clouds. For all three models we can find combinations of parameters that reproduce the CS profiles reasonably well provided that the component clouds have a core-envelope structure with a temperature gradient. Cores with high temperature and high molecular hydrogen density are needed to match the higher transitions (e.g. J=7-6) observed towards A and G. The lower temperature, low density gas needed to create the inverse P-Cygni profile seen in the CS J=2-1 line (with 5" beam) towards H II region G arises from different components in the 3 models. The infalling envelope of cloud G plus cloud B creates the absorption in global collapse, cloud B is responsible in local collapse, and a separate cloud, G', is needed in the case of many static clouds. The exact nature of the velocity field in the envelopes for the case of local collapse is not important as long as it is in the range of 1 to 5 km/s for a turbulent velocity of about 6 km/s. High resolution observations of the J=1-0 and 5-4 transitions of CS and C34S may distinguish between these three models. Modeling existing observations of HCO+ and C18O does not allow one to distinguish between the three models but does indicate the existence of a bipolar outflow.Comment: 42 pages, 27 figures, accepted for publication in the ApJS August 2004, v153 issu