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Volume 106, Issue 2, 08 January 1997

A threedimensional wavepacket method for the CH overtone spectroscopy and intramolecular vibrational relaxation dynamics of the fluoroform molecule
View Description Hide DescriptionThe overtone spectroscopy and intramolecular vibrational relaxation dynamics of CH chromophore in the fluoroform molecule is studied by a threedimensional (3D) timedependent wavepacket method, and the results are compared with the experiment and with those of a 2D (stretch–bend) wavepacket method. A third mode (CF symmetrical stretch) is included in order to explain the long time dynamics and the combination bands between the CF stretch fundamental and the Fermi polyad frequencies. The comparison with the 2D study is carried out by the use of a full set of dynamical and spectroscopic variables, based on the autocorrelation function of the bright states of each polyad. The spectroscopic variables then follow by Fourier transforming the autocorrelation function, while the dynamical ones emerge via survival probability in the frame of the dynamical statistical ensemble. These include several relaxation times and the number of cells and rates of phase–space exploration. The specific effect of the third mode is monitored by following the reduced dynamics of the system irrespective of the polyad stretch–bend dynamics, through population evolution. Dynamical results clearly reveal the third mode effects at very short and long times. In the last regime, we can observe a great span of different behaviors, depending on how the third mode excited states are involved. This richer variety of dynamical patterns cannot be observed in a twomode model and justifies the present work. The spectroscopic results of both models are in good agreement with the experimental results.

Electronic dephasing and electron–phonon coupling of aluminum phthalocyanine tetrasulphonate in hyperquenched and annealed glassy films of ethanol and methanol over a broad temperature range
View Description Hide DescriptionThe electronic dephasing (spectral dynamics) and electron–phonon coupling of aluminum phthalocyanine tetrasulphonate (APT) in glassy films of ethanol and methanol were investigated by nonphotochemical hole burning over a broad temperature range, ∼5–100 K. Films formed by hyperquenching (∼10^{6} K s^{−1}) at 4.7 K were studied as well as films that were subsequently annealed at temperatures up to ∼170 K. Results are compared against those for APT in glassy water [Kim et al., J. Phys. Chem. 99, 7300 (1995); Reinot et al., J. Chem. Phys. 104, 793 (1996)]. As in the case of water, the linear coupling is weak with a Huang–Rhys factor S∼0.4 but the mean phonon frequencies for ethanol and methanol of 26 and 17 cm^{−1} are considerably lower than the 38 cm^{−1} value for water. These modes are assigned as pseudolocalized with significant amplitude (libration) localized on APT. Below about 8 K, the electronic dephasing/spectral diffusion is dominated by coupling to the tunneling intrinsic twolevel systems of the glass. At higher temperatures the electronic dephasing is dominated by the exchange coupling mechanism, which derives from diagonal quadratic electron–phonon coupling. Here, for both ethanol and water, a pseudolocalized mode(s) at ∼50 cm^{−1} is operative. This frequency corresponds to a peak in the spectral density of the liquids which for water is due to the transverse acoustic mode. The results show that the modes responsible for linear and quadratic coupling are distinctly different. Implications of this for optical coherence loss in liquids are considered. Novel results from annealing experiments are reported and discussed in terms of the complex phase diagrams of ethanol and methanol. Formation of the glass from the supercooled liquid just above the melting point of a crystalline phase leads to a marked reduction (∼10×) in the homogeneous width of the zerophonon hole at 4.7 K. This is interpreted in terms of a reduction in the density of intrinsic twolevel systems due to reduced structural disorder of the glass formed from the supercooled liquid. As in the case of water, the highly efficient hole burning in glassyethanol and methanol is observed to become highly inefficient upon formation of a crystalline phase as predicted by the Shu–Small mechanism for nonphotochemical hole burning. The close connection between this mechanism and Onsager’s inverse snowball effect for solvent dynamics around an instantaneously created point charge or dipole in a liquid is emphasized.

Nuclear magnetic resonance microscopy in liquids using the dipolar field
View Description Hide DescriptionWe demonstrate theoretically and experimentally how the dipolar field can be used in nuclear magnetic resonance(NMR) to investigate the structure of heterogeneous liquid systems. Using the Fourier transformed dipolar field and magnetization distribution, a simple relation between the NMR signal generated by the dipolar field and the sample structure can be established. On the basis of this relation, theoretical models for periodic structures have been derived and used to analyze the variation of the NMR signal as a function of the spatial modulation imposed on the magnetization. If the spatial modulations imposed on the transverse and longitudinal magnetizations have the same wavelength, the signal generated by the dipolar field is a continuous function of the modulation wavelength and is sensitive to the structure of the sample. When this condition is not met, diffraction phenomena may be possible in periodic structures. To test the theoretical work, experimental data have been obtained from water surrounding randomly packed microspheres. These data are in agreement with the theoretical predictions and show that a resolution of the order of 10 μm can be achieved for highly mobile systems. For spin bearing molecules, whose selfdiffusion coefficient is two orders of magnitude less than that of free water, submicrometer resolution is expected.

Spectroscopy of pendular states: Determination of the electric dipole moment of ICl in the X ^{1}Σ^{+}(v″=0) and A ^{3}Π_{1}(v′=6–29) levels
View Description Hide DescriptionIn this paper, we describe new measurements of the electric dipole moment of ICl A and X states. These measurements are deduced from a Doppler free laser induced fluorescencespectroscopy of the AX transition and concern the X v′′=0 and 17 vibrational levels of the A state. A complete modelization of the spectra has been developed and the accuracy of these measurements is rather high. We have been able to invert in an approximate manner the vibrational dependence of the A state dipole moment to get its dependence with the internuclear distance r. The observed dependence at large r is well rationalized by the model of an ionic–covalent avoided crossing, and appears to be very close to the result of an ab initio calculation of the same state of IF molecule.

Spectroscopic characterization of the AsF_{2} radical
View Description Hide DescriptionThe electronic spectrum of the AsF_{2} radical was observed between 330 and 380 nm using onecolor, massresolved, 2+1 resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectrum arises from twophoton resonances with the F̃ (T _{0}=54 355±15 cm^{−1}, cm^{−1}, and cm^{−1}) and H̃ (T _{0}=57 480±15 cm^{−1}, cm^{−1}, and cm^{−1}), Rydberg states. A third photon ionized the radicals. The REMPI spectrum exhibits vibrational hot bands that give spectroscopic constants for AsF_{2} (X̃ ^{2}B_{1}) of =± cm^{−1} and =± Solutions of the Rydberg equation indicate that plausible assignments for the F̃ and H̃ states are limited to the set of 4d and 5pRydberg states.

The vibrational spectra of molecular ions isolated in solid neon. XIV. , , , and their ylidion isomers
View Description Hide DescriptionWhen a Ne: sample is codeposited at approximately 5 K with neon atoms that have been excited in a microwavedischarge, the infrared spectrum of the resulting solid deposit includes prominent absorptions which can be assigned to cation products. It has previously been established that the ylidion isomers have stable potential minima and that is lower in energy than . The identification of the new absorptions is aided by experimental studies on isotopically substituted Ne: samples and by ab initio calculations of the structures and vibrational fundamentals of both and . In each of the three systems, a prominent absorption which is intermediate in frequency between the gasphase (and neonmatrix) absorptions of HX and can be assigned to the ylidion, as can be several other absorptions. Still other absorptions can be assigned to and . The behavior of the product absorptions on exposure of the deposit to filtered visible and ultraviolet radiation is consistent with the proposed assignments.

Pure rotational spectrum, quadrupole coupling constants and structure of the dimer of pyrrole
View Description Hide DescriptionRotational transitions of a pyrrole dimer have been identified and measured over the 8–18 GHz range using a pulsednozzle Fouriertransform microwave spectrometer. In addition to the parent species, nine and D isotopomers have been analyzed. Apart from the and D nuclear quadrupole splittings, the rotational transitions did not show any additional splittings arising from large amplitude motions. Rotational constants, centrifugal distortion constants and, for two isotopomers, quadrupole coupling constants have been fitted to the measured frequencies of rotational transitions. The observed rotational constants are consistent with essentially a Tshaped structure for the dimer. From the rotational constants of all isotopomers, a partial structure of the dimer has been determined. Three structural parameters have been fitted to the differences in the planar moments of inertia between the isotopically substituted species and the parent species. The planes of the two pyrrole monomers form an angle of 55.4(4) with the nitrogen side of one ring directed to the electron system of the other ring establishing a weak hydrogen bond. The centrifugal distortion constants of the dimer of pyrrole have been used to estimate the frequency of the van der Waals stretching mode and the dissociation energy of the complex in a pseudodiatomic approximation.

Temperature dependence of the vibrational relaxation processes in natural and isotopically pure ^{32}S_{8}: Effect of the isotopic impurities on infrared phonon lifetimes
View Description Hide DescriptionWe have measured the temperature dependence of the infrared bandwidth of several lattice phonons and internal vibrons in natural and ^{32}S isotopically pure orthorhombic sulfur crystals, using a high resolution FTIRinterferometer. The experimental data were analyzed in terms of anharmonic phonon–phonon coupling processes and of contributions to the bandwidth due to the presence of randomly distributed isotopic impurities. The same threephonon processes that contribute to the lifetime of the Raman bands were found to be active also for the infrared ones in the isotopically pure crystal. In parallel to the increase of the twophonon density of states, the thirdorder anharmonic coupling coefficients for phonon decay processes were found to increase from the lowest to the highest lattice mode. The effect of the isotopic impurities was explained in terms of two contributions, the first, which is temperature independent, due to pure harmonic scattering processes and the second due to the combined effect of anharmonicity and scattering processes, which is instead dependent on the temperature. The contribution due to the presence of impurities was found to dominate the width of the lattice modes in the natural crystal at low temperatures. This purely harmonic contribution can be in some cases almost one order of magnitude larger than that due to the normal anharmonic decay processes in the isotopically pure crystal. Only the width of the factor group component of infrared and Raman vibrons close to the frequency of the isolated molecule was found to be affected by the presence of impurities through a resonant scattering mechanism.

Vibrational cooling after ultrafast photoisomerization of azobenzene measured by femtosecond infrared spectroscopy
View Description Hide DescriptionThe vibrational cooling of azobenzene after photoisomerization is investigated by time resolvedIR spectroscopy with femtosecond time resolution. Transient difference spectra were obtained in a frequency range where phenyl ring modes and the central N=Nstretching mode absorbs. The experimental data are discussed in terms of a simple theoretical model which was derived in order to account for the offdiagonal anharmonicity between the investigated highfrequency modes and the bath of the remaining lowfrequency modes in a polyatomic molecule. It is shown that these offdiagonal anharmonic constants dominate the observed transient absorbance changes while the anharmonicity of the highfrequency modes themselves (diagonal anharmonicity) causes only minor effects. Based on the transient IR spectra, the energy flow in the azobenzene molecule can be described as follows: After an initial ultrafast intramolecular energy redistribution process, the decay of the related intramolecular temperature occurs via intermolecular energy transfer to the solvent on a time scale of ca. 20 ps.

Mobility and formation kinetics of (NH_{3}) _{n} cluster ions (n=0–3) in helium and helium/ammonia mixtures
View Description Hide DescriptionNH_{4} ^{+}(NH_{3}) _{n} (n=0–3) cluster ions are produced in a fieldfree flow tube section of a selected ion flow–drift tube (SIFDT) apparatus. Cluster ion mobilities are measured in mixtures of He and NH_{3} and used to obtain the individual mobilities in helium and in ammonia by applying Blanc’s law to the mixtures. Mobilities of the cluster ions are also measured in pure helium by producing the ions in the ion source of a flowing afterglow, selected ion flow–drift tube apparatus (FASIFDT). The measurements in pure helium compare well with the mobilities in helium obtained by applying Blanc’s law to the mixtures. The zero field mobilities of the cluster ions in helium are 22.1±0.4 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}, 16.6±0.4 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}(NH_{3}), 12.2±0.4 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}(NH_{3})_{2}, and 12.1±0.4 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}(NH_{3})_{3}. The decrease with increasing size of the cluster can be explained in terms of the sizes of the geometric cross sections. The zerofield mobilities in NH_{3} are 0.94±0.35 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}, 0.83±0.22 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}(NH_{3}), 0.50±0.27 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}(NH_{3})_{2}, and 0.25±0.20 cm^{2} V^{−1} s^{−1} for NH_{4} ^{+}(NH_{3})_{3}. The small values of the mobilities in these polar gas systems are understood in terms of the strong ion–dipole interactions. Calculated mobilities in NH_{3} are obtained by computing the collision cross section with the ion–dipole interactions taken into account; the results compare well with the measurements for NH_{4} ^{+} and NH_{4} ^{+}(NH_{3}). However, the measured mobilities of the larger cluster ions are smaller than the computed values. The discrepancies may be due to several factors including dipole–dipole interactions, ligand exchange reactions,formation of longlived quasibound complexes, and efficient transfer of kinetic energy into internal energy of the cluster ion and the ammonia molecules.

Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm
View Description Hide DescriptionWe have studied the photodissociation of dimethyl sulfoxide, DMSOh_{6} and DMSOd_{6}, at 193 nm using the technique of photofragment translational spectroscopy with a tunable vacuum ultraviolet product probe provided by undulator radiation on the Chemical Dynamics Beamline at the Advanced Light Source. In contrast to previous investigations we have found the dissociation to proceed via a stepwise mechanism involving multiple reaction channels. The primary dissociation, S–C bond cleavage to eliminate a methyl radical, was found to have two competing channels with distinct translational energy distributions. The translational energy distribution for the major primary dissociation channel suggests that it proceeds in a statistical manner on the ground electronic surface following internal conversion. In competition with this channel is a primary dissociation that exhibits a translational energy distribution suggestive of dissociation on an excited electronic surface with most of the available energy partitioned into translational and electronic degrees of freedom. Secondary decomposition of the CD_{3}SO intermediate was found to proceed exclusively via C–S bond cleavage, CD_{3}SO→CD_{3}+SO. However, secondary decomposition of the CH_{3}SO intermediate was found to exhibit competition between CH_{3}SO→CH_{3}+SO and CH_{3}SO→CH_{2}SO+H. The dissociation to CH_{3} and SO was the major secondary decomposition channel with the translational energy distribution indicating a barrier to recombination of >8 kcal/mol. While a minor hydrogen atom elimination channel was found to play a role in secondary decomposition of CH_{3}SO intermediates, no analogous secondary C–D bond cleavage was detected from the CD_{3}SO intermediates indicating the importance of tunneling in the secondary decomposition of CH_{3}SO.

The cumulative reaction probability for the H + OH reaction
View Description Hide DescriptionThe cumulative reaction probability [CRP or for the fouratom reaction, H+OH→H+HO is calculated using one of the formulations of Miller, Schwartz, and Tromp [J. Chem. Phys. 79, 4889 (1983)] and the transition state wave packet (TSWP) approach of Zhang and Light [J. Chem. Phys. 104, 6184 (1996)]. It is shown that locating the dividing surface of the flux operator in the transition state region significantly reduces the number of wave packets which must be followed in order to converge the CRP as compared to the use of initial state selected wave packets (ISSWP). In addition we examine the use of transition state normal coordinates (versus Jacobi coordinates) and show that the use of transition state wave packets defined in normal coordinates yields more rapid convergence of the CRP and individual contributions of the TSWP to the CRP can closely approximate the probabilities of reaction for each transition state as a function of energy. Problems with large amplitude motions using the normal coordinates of the loose nonlinear transition state are shown to be absent if normal coordinates of a linear transition state are used. Applications to the 3D H + H = 0) reaction and to the 6D H + OH = 0) reaction demonstrate that both and the initial state reaction probabilities at many energies can be evaluated accurately and efficiently by propagation of each TSWP only once.

Absolute stateselected total cross sections for the ion–molecule reactions O^{+} (^{4}S,^{2}D,^{2}P)+H_{2}(D_{2})
View Description Hide DescriptionAbsolute total cross sections for the stateselected reactions of O^{+} (^{4}S,^{2}D,^{2}P)+H_{2} (D_{2}) have been measured in the centerofmass collision energy (E _{c.m.}) range of 0.02–12 eV. The cross sections for OH^{+} (OD^{+}) from O^{+}(^{2} D)+H_{2} (D_{2}) are slightly higher than those from O^{+}(^{4} S)+H_{2} (D_{2}), whereas the OH^{+} (OD^{+}) cross sections from O^{+} (^{2} P)+H_{2} (D_{2}) are ≈40% lower than those from O^{+}(^{4} S)+H_{2} (D_{2}) and O^{+} (^{2} D)+H_{2} (D_{2}). At E _{c.m.}<0.5 eV, the total cross sections for OH^{+} (OD^{+}) from O^{+} (^{4} S)+H_{2} (D_{2}) and O^{+}(^{2} D)+H_{2} (D_{2}) are in accord with those predicted by the Langevin–Gioumousis–Stevenson model. Significantly higher cross sections are observed for H^{+} (D^{+}) and H_{2} ^{+} (D_{2} ^{+}) from O^{+}(^{2} D)+H_{2} (D_{2}) and O^{+}(^{2} P)+H_{2} (D_{2}), as compared to those from O^{+}(^{4} S)+H_{2} (D_{2}). The exothermic nature of the O^{+} (^{2}D,^{2}P)+H_{2} (D_{2}) charge transfer collisions accounts for the high cross sections observed for H_{2} ^{+} (D_{2} ^{+}). While the H^{+} (D^{+}) ions observed in the O^{+}(^{4} S)+H_{2} (D_{2}) reaction are identified with the H^{+} (D^{+})+O+H channel, the H^{+} (D^{+}) ions from the reactions involving O^{+}(^{2} D) and O^{+}(^{2} P) are associated mostly with the H^{+} (D^{+})+OH (OD) channel, the formation of which obeys the spinconservation rule. The comparison of the sum (σ _{T} ) of cross sections for OH^{+} (OD^{+}), H_{2} ^{+} (D_{2} ^{+}), and H^{+} (D^{+}) from O^{+}(^{4} S)+H_{2} (D_{2}) to those from O^{+}(^{2} D)+H_{2} (D_{2}) and O^{+}(^{2} P)+H_{2} (D_{2}) shows that the σ _{T} s for O^{+}(^{4} S)+H_{2} (D_{2}), O^{+}(^{2} D)+H_{2} (D_{2}), and O^{+}(^{2} P)+H_{2} (D_{2}) at E _{c.m.}<0.5 eV are comparable. At E _{c.m.}>0.5 eV, the σ _{T} s for O^{+}(^{2} P)+H_{2} (D_{2}) are greater than those for O^{+}(^{2} D)+H_{2} (D_{2}), which in turn are greater than those for O^{+}(^{4} S)+H_{2} (D_{2}). This observation is attributed to the increase in the number of accessible product channels for reactions involving the excited O^{+}(^{2} D) and O^{+}(^{2} P) reactant ions.

The zeropoint energy problem in classical trajectory simulations at dissociation threshold
View Description Hide DescriptionQuasiclassical trajectory calculations offer a costeffective means of investigating the dynamics of chemical reactions. However, they suffer from the zeropoint energy (ZPE) problem, whereby the (quantum) ZPE motion can contribute to an overestimation of the rate coefficient. This paper reports on some dynamics of the Hénon–Heiles system. Dynamics of the water molecule at energies just below the (quantum) dissociation threshold, are also reported. The TRAPZ method [Lim and McCormack, J. Chem. Phys. 102, 1705 (1995)] leads to a definite improvement over unconstrained classical mechanics.

Quantum dynamics of the Walden inversion reaction Cl^{−}+CH_{3}Cl→ClCH_{3}+Cl^{−}
View Description Hide DescriptionQuantum scattering calculations on the S _{N}2 reaction Cl^{−}+CH_{3}Cl→ClCH_{3}+Cl^{−} are reported. The rotating bond approximation (RBA) has been adapted so that three degrees of freedom including the C–Cl stretching vibration and the CH_{3} umbrella mode are treated explicitly. The calculations have been done with minor modifications of a potential due to Vande Linde and Hase. It is found that initial excitation of the C–Cl vibration has a large effect on the reaction probabilities, while excitation of the CH_{3} umbrella vibration is less significant. The reaction is dominated by scattering resonances with lifetimes ranging from 0.1 to 10 ps. It is found that the length of the C–Cl bond at the transition state of the reaction has a particularly pronounced effect on the reaction probabilities. The magnitude of the quantum reaction probabilities compares quite well with those calculated using the quasiclassical trajectory method.

A sequential formula for electronic coupling in long range bridgeassisted electron transfer: Formulation of theory and application to alkanethiol monolayers
View Description Hide DescriptionA recursion relation is formulated for the Green’s function for calculating the effective electron coupling in bridgeassisted electronic transfer systems, within the framework of the tightbinding Hamiltonian. The recursion expression relates the Green’s function of a chain bridge to that of the bridge that is one unit less. It is applicable regardless of the number of orbitals per unit. This method is applied to the system of a ferrocenylcarboxyterminated alkanethiol on the Au(111) surface. At larger numbers of bridge units, the effective coupling strength shows an exponential decay as the number of methylene(–CH_{2}–) units increases. This sequential formalism shows numerical stability even for a very long chain bridge and, since it uses only small matrices, requires much less computer time for the calculation. Identical bridge units are not a requirement, and so the method can be applied to more complicated systems.

Dimensional perturbation theory for Regge poles
View Description Hide DescriptionWe apply dimensional perturbation theory to the calculation of Regge pole positions, providing a systematic improvement to earlier analytic firstorder results. We consider the orbital angular momentum as a function of spatial dimension for a given energy , and expand in inverse powers of . It is demonstrated for both bound and resonance states that the resulting perturbation series often converges quite rapidly, so that accurate quantum results can be obtained via simple analytic expressions given here through third order. For the quartic oscillator potential, the rapid convergence of the present series is in marked contrast with the divergence of the more traditional dimensional perturbation series, thus offering an attractive alternative for bound state problems.

Coupled oscillators as a model for vibrations of polyatomic molecules
View Description Hide DescriptionThe system of two deformed oscillators coupled so that the total Hamiltonian has the su(2) symmetry is proved to be equivalent, to lowest order approximation, to a system of two identical Morse oscillators coupled by the crossanharmonicity usually used empirically in describing vibrational spectra of triatomic molecules. The symmetry also imposes a connection between the selfanharmonicity of the Morse oscillators and the crossanharmonicity strength, which can be removed by replacing the oscillators by deformed anharmonic oscillators. The generalization to oscillators is straightforward. The applicability of the formalism to highly symmetric polyatomic molecules is discussed.

Twodimensional transport and wall effects in the thermal diffusion cloud chamber. I. Analysis and operations criteria
View Description Hide DescriptionIn this paper we present results of a twodimensional (z,r) treatment of the mass and energy transfer processes that occur during the operation of a thermal diffusion cloud chamber. The location of the wall is considered in solving the mass and energy transport equations, in addition to the vertical distance, z, between the upper and lower plate surfaces. We examine the effect of aspect (diameter to height) ratio on chamber operation; the effects of operation with either a dry or a wet interior chamber wall on temperature, supersaturation, nucleation rate, and total density profiles in the chamber; the effect of overheating the interior of the chamber wall on these conditions within the cloud chamber; and the effects associated with using different density background gases on the operation of the chamber. In a second paper, immediately following, we apply the formalism and the solutions developed in this paper to address the important problem of buoyancydriven convection that can accompany (seemingly normal) operation of thermal diffusion cloud chambers in nearly all ranges of total pressure and temperature.

Twodimensional transport and wall effects in the thermal diffusion cloud chamber. II. Stability of operation
View Description Hide DescriptionIn this paper, the second of a series of two presenting a detailed description of thermal diffusion cloud chamber operation, we address the operational stability of the vapor–gas mixture in a diffusion cloud chamber with respect to density profile extrema and the accompanying possibility of buoyancydriven convective flow disturbances. We examine conditions for stable operation (no convective flow disturbances) in the central portion of the cloud chamber, as well as conditions necessary for stable operation in the vicinity of the cloud chamber wall. We find that the total density profile in the central portion of the cloud chamber can pass through a density minimum even though the density at the upper plate surface is less than the density at the lower plate surface. This local density profile inversion can result in unstable (convective) behavior that propagates through the cloud chamber. Furthermore, we find that local extrema in the total density profile near the chamber wall can lead to subtle, convective flows that are difficult to detect yet can exert a profound influence on nucleation in the central portion of the cloud chamber. We have developed a simple method to estimate the limiting total pressure in a cloud chamber that will support stable operation. From results of our investigations based upon this method, it appears that the thermal diffusion cloud chamber is best suited for experiments at higher temperatures where the accessible total pressure range is largest. Finally, we find that results of our investigation into the effects of total pressure and kind of background gas on nucleation in diffusion cloud chambers involving the low molecular weight alcohols and hydrogen and helium background gases cannot be explained on the basis of these kind of density disturbances occurring within the diffusion cloud chamber. Also, for (relatively) low vapor pressure materials, such as 1pentanol or other high molecular weight alcohols and alkanes stability limitations may preclude nucleationmeasurements at low temperatures using a diffusion cloud chamber altogether.