Time-independent Constant Research Articles

Challenges for superstring cosmology

We consider whether current notions about superstring theory below the Planck scale are compatible with cosmology. We find that the anticipated form for the dilaton interaction creates a serious roadblock for inflation and makes it unlikely that the universe ever reaches a state with zero cosmological constant and time-independent gravitational constant.

Bunching and Antibunching Effects in a System ofNTwo-level Atoms

Abstract The problem of exhibiting bunching and antibunching phenomena for a system of N two-level atoms in interaction with light is investigated. The conditions for the change from bunching to antibunching and vice versa are obtained for resonance, and for time-independent and time-dependent coupling constant. The conditions for the case of resonance and time-independent coupling constant are investigated for both generalized coherent states and squeezed coherent states.

Are vibrational relaxation times really constant? I. The vibrational relaxation of N 2O

The vibrational relaxation of pure N 2O was studied behind shock waves in the temperature range 450–1750 K using a laser beam deflection technique. It is shown that the phenomenological relaxation time,τ′, is not a time-independent constant. Below 1200 K it increases with time; above 1200 K it decreases with time. The many disparate results in the literature are reconciled in terms of a mechanism whereby the step N 2O(0, 0, 0) → N 2O(0, 1, 0) is rate determining at low temperatures when the ν 3 mode is not involved, and the parallel step N 2O(1, 0, 0) + N 2O(0, 0, 0) → N 2O(0, 2, 0) + N 2O(0, 1, 0) takes over at high temperatures close to equilibrium. Rate constants for these processes are extracted from the limiting far-from equilibrium relaxation time, i.e. from τ′(0), where log pτ′(0) = 11.0 T −1 3 −1.67 (in atm μs), and from the limiting near-equilibrium relaxation time, i.e. from τ′(∞), where log pτ′(∞) = 19.4 T −1 3 −2.46 (in atm μs). The relaxation process was simulated by solving the master equation numerically. All energy levels up to 5930 cm −1 connected by all possible VT and inter- and intra-molecular VV processes were included, subject to the restriction that the energy gap ¦Δϵ¦ 700 cm −1. Microscopic rate coefficients, scaled according to ϵ exp(− C ¦Δϵ¦), were assigned, reducing the problem to one of three parameters: C (for VT processes), C VV (for VV processes) and k 01 10/ k 10, where k 10 is the rate constant for N 2O(0, 1, 0) → N 2O(0, 0, 0) and k 01 10 that for N 2O(0, 0, 0) + N 2O(0, 1, 0) → N 2O(0, 0, 0)+N 2O(0,1,0). Computer simulations of shock-induced and laser-induced relaxation are made. They are interpreted in terms of the three parameters. There was semi-quantitative agreement with our experimental results, which are consistent with values for C = 0.0038 cm at room temperature; C VV = 0.021 → 0.0065 cm, and k 01 10/ k 10 ⩽ 5000 → 12.7 in the range 300 → 1800 K. It was further found that intermolecular VV processes are responsible for the time dependence of τ′, and that the increasing density of states accessible at longer times and at higher temperatures is responsible for the change in mechanism.