Philip R. Brooks

Rice University, Houston, TX, United States

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Publications (25)120.17 Total impact

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    Peter W Harland, Philip R Brooks
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    ABSTRACT: Fast potassium atoms donate an electron to CCl(3)NO(2) molecules to form K(+) ions and the negative ions O(-), Cl(-), NO(2) (-), CCl(3) (-), CCl(2)NO(2) (-), CCl(3)NO(-), and CCl(3)NO(2) (-). Threshold energies are measured for these ions and electron affinities for CCl(2)NO(2) (-), CCl(3)NO(-), and CCl(3)NO(2) (-) are estimated to be 2.35, 2.35, and 1.89 eV (+/-0.6 eV), respectively. The threshold energies show that the C-N and N-O bonds are substantially weaker than in nitromethane. The CCl(3)NO(2) molecules are oriented before the collision and at energies near 2.5 eV the electron appears to transfer to the pi( *) (NO) orbital forming the parent negative ion, CCl(3)NO(2) (-), which is stabilized by interacting with the K(+) donor. As the collision energy increases the parent negative ion fragments and the orientation dependence of the fragment ions helps understand the fragmentation pathway.
    The Journal of Chemical Physics 01/2010; 132(4):044307. · 3.16 Impact Factor
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    Philip R Brooks
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    ABSTRACT: Electron transfer collisions between neutral K atoms and neutral, oriented trifluoroacetic acid molecules, CF(3)CO(2)H, are studied in crossed molecular beams at center of mass energies from 6 to 18 eV. An electron transfer produces a pair of ions with enough energy to escape the Coulomb attraction, and the ions are detected in separate time-of-flight mass spectrometers. The principle ions formed are K(+) and the trifluoroacetate ion, CF(3)CO(2)(-) ion, and this channel is favored for attack at the positive (-CO(2)H) end of the molecule. The steric asymmetry suggests that the electron is transferred into the pi*(CO) orbital. The nascent K(+) perturbs the molecular symmetry, allowing electron migration to the sigma*(OH) orbital to break the O-H bond and form CF(3)CO(2)(-).
    The Journal of Physical Chemistry A 12/2009; 113(52):14296-301. · 2.77 Impact Factor
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    Philip R Brooks
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    ABSTRACT: Electron transfer from K atoms to oriented acetic acid molecules produces acetate ions (and K(+)) when the CO(2)H end of the molecule is attacked. The electron enters the pi(CO)(*) orbital and the donor atom distorts the molecule to allow migration to the sigma(OH)(*) orbital, thereby breaking the bond.
    The Journal of Chemical Physics 05/2009; 130(15):151102. · 3.16 Impact Factor
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    ABSTRACT: Electron transfer from K atoms to oriented CH3CN, CH3NC, and CCl3CN is studied in crossed beams at energies near the threshold for forming an ion pair. For the methyl compounds, the dominant ions are K+ and CN-; the steric asymmetry is very small and energy-independent, characteristic of sideways attack with the electron apparently entering the pi*CN antibonding orbital. Migration of the electron to the sigma*CC orbital to break the C-C bond is greatly facilitated by interaction with the atomic donor. CH2CN- is formed in collisions preferring CH3-end attack, and the steric asymmetry becomes very large near threshold. CCl3CN mostly forms Cl- in collisions slightly favoring the CCl3 end with a small energy dependence with the electron apparently entering the sigma* LUMO. CN- is formed in much smaller yield with a slight preference for the CN end. The parent negative ion CCl3CN- is observed, and a lower limit for its electron affinity is estimated to be 0.3 eV. Fragment ions CCl2CN- and CClCN- are also observed with upper limits for the quantity bond dissociation energy - electron affinity (BDE - EA) estimated to be 0.6 and 1.0 eV, respectively.
    Journal of the American Chemical Society 01/2008; 129(50):15572-80. · 10.68 Impact Factor
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    Philip R Brooks, Peter W Harland, Crystal E Redden
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    ABSTRACT: Electrons are transferred in collisions between potassium atoms and CH(3)NO(2) molecules that have been oriented in space prior to collision. The electron transfer produces K(+) ions, parent negative ions CH(3)NO(2)(-), and the fragment ions e(-), NO(2)(-), and O(-) in amounts that depend on the energy. The positive and negative ions are detected in coincidence by separate time-of-flight mass spectrometers at various collision energies for both CH(3)-end attack and NO(2)-end attack. The steric asymmetry for electrons and CH(3)NO(2)(-) is essentially zero, but the steric asymmetry for NO(2)(-) shows that NO(2)(-) is formed mainly in CH(3)-end collisions. There is evidence that the electrons and NO(2)(-) have the same transient precursor, despite having different steric asymmetries. It appears likely that the precursor is formed by electron transfer mainly in collisions normal to the molecular axis leading to near zero steric asymmetry for the electron. This transient precursor can also eject an NO(2)(-) ion, which is more likely to be removed as KNO(2) salt when K(+) ions are near the NO(2) end of the molecule, with the result that CH(3)-end collisions seem to produce more NO(2)(-).
    The Journal of Physical Chemistry A 05/2006; 110(14):4697-701. · 2.77 Impact Factor
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    Philip R Brooks, Peter W Harland, Crystal E Redden
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    ABSTRACT: Beams of sodium atoms with energies of a few eV are crossed with a beam of oriented CH3NO2 molecules to study the effect of collision energy and orientation on electron transfer. The electron transfer produces Na+ ions and free electrons, parent negative ions (CH)NO2-), and fragmentation ions NO2- and O- in proportions that depend on the collision energy. The steric asymmetry is very small or zero and suggests that production of all of the ions is favored by sideways attack with respect to the permanent dipole along the C-N axis. In these experiments, the electron appears to be transferred into the 2B1 state of the anion comprising mainly the pi*NO LUMO, producing a valence-bound state rather than a dipole-bound state.
    Journal of the American Chemical Society 05/2006; 128(14):4773-8. · 10.68 Impact Factor
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    ABSTRACT: Rubidium atoms are accelerated in a high-temperature expansion of hydrogen to produce beams with energies high enough to observe collisional ionization with a cross beam. The speed of the atoms is directly measured by time-of-flight techniques, and the positive and negative ions produced are detected in separate mass spectrometers and detected in coincidence. Chloroperfluoroethane produces C(2)F(5)(-) and Cl(-) ions, whereas iodoperfluoroethane produces I(-), C(2)F(5)(-), and C(2)F(5)I(-) ions. When the measured speed distributions are used, the signal versus energy may be deconvolved to yield thresholds and electron affinities (EAs). The EA for C(2)F(5)I is measured to be 0.96 +/- 0.1 eV. Anomalously high EA values result for C(2)F(5) apparently because C(2)F(5)(-) is produced by parts per million concentrations of Rb(2).
    The Journal of Physical Chemistry A 11/2005; 109(41):9213-9. · 2.77 Impact Factor
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    Peter W Harland, Philip R Brooks
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    ABSTRACT: Different alkali metal atoms are observed to donate electrons to CF(3)Br molecules that are oriented in space. For collision energies high enough to overcome the Coulomb attraction, a positive ion/negative ion pair is observed and mass-analyzed using coincident time-of-flight mass spectroscopy. The alkali metal cation and various negative ions are observed. The most abundant negative ion is the bromide ion, Br(-), formed preferentially by attack at the Br end of the molecule. The steric asymmetry to produce Br(-) is almost identical for all of the alkali metal donors. Fluoride ions are formed in smaller abundance and reflect completely different behavior among the donors. Sodium and potassium have high thresholds and prefer the CF(3) end of the molecule, whereas cesium prefers the Br end of the molecule. Sodium and potassium apparently interact with the transient CF(3)Br(-) molecular negative ion while it is in the process of decomposing.
    Journal of the American Chemical Society 10/2003; 125(43):13191-7. · 10.68 Impact Factor
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    Beike Jia, Jonathan Laib, R F M Lobo, Philip R Brooks
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    ABSTRACT: Beams of hyperthermal K atoms cross beams of the oriented haloforms CF(3)H, CCl(3)H, and CBr(3)H, and transfer of an electron mainly produces K(+) and the X(-) halide ion which are detected in coincidence. As expected, the steric asymmetry of CCl(3)H and CBr(3)H is very small and the halogen end is more reactive. However, even though there are three potentially reactive centers on each molecule, the F(-) ion yield in CF(3)H is strongly dependent on orientation. At energies close to the threshold for ion-pair formation ( approximately 5.5 eV), H-end attack is more reactive to form F(-). As the energy is increased, the more productive end switches, and F-end attack dominates the reactivity. In CF(3)H near threshold the electron is apparently transferred to the sigma(CH) antibonding orbital, and small signals are observed from electrons and CF(3)(-) ions, indicating "activation" of this orbital. In CCl(3)H and CBr(3)H the steric asymmetry is very small, and signals from free electrons and CX(3)(-) ions are barely detectable, indicating that the sigma(CH) antibonding orbital is not activated. The electron is apparently transferred to the sigma(CX) orbital which is believed to be the LUMO. At very low energies the proximity of the incipient ions probably determines whether salt molecules or ions are formed.
    Journal of the American Chemical Society 12/2002; 124(46):13896-902. · 10.68 Impact Factor
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    Philip R Brooks, Sean A Harris
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    ABSTRACT: Electron transfer collisions between beams of neutral K atoms and neutral alkyl bromide R–Br molecules (RCH 3 ,t-C 4 H 9) are observed by detecting positive and negative ions in coincidence for energies 4 eV, the minimum energy for overcoming the Coulomb attraction between ions. The molecules are state selected by a hexapole electric field and oriented prior to the electron transfer. The steric asymmetry for both molecules above 6 eV shows that ''frontside,'' or Br end attack, is favored to form Br , with t-C 4 H 9 Br being more asymmetric than CH 3 Br. The asymmetry maximizes near 5 eV and as the energy decreases, apparently changes sign to favor ''backside,'' or alkyl-end attack. Free electrons and K are detected from t-C 4 H 9 Br and show a similar change in preferred orientation: at low energies alkyl end attack is favored, and at high energies Br end is favored. These observations suggest that the electron is transferred into different orbitals with different spatial distributions as the energy is varied. Steric factors are evaluated from the experimental data. The steric factor for t-C 4 H 9 Br is generally smaller than for CH 3 Br and above about 5 eV, both increase with energy in Arrhenius-type dependence. The apparent ''steric activation energy'' is 2.2 eV for CH 3 Br and 3.9 eV for t-C 4 H 9 Br. © 2002 American Institute of Physics.
    The Journal of Chemical Physics 09/2002; · 3.16 Impact Factor
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    ABSTRACT: K{sup +} ions have been detected from the intersection of a beam of K atoms (5-30 eV) with beams of various simple molecules, such as CHâBr and CFâBr, which had been oriented prior to the collision. Production of ions in the collision is found to be highly dependent on orientation. The effect is most pronounced near threshold (â5 eV) and almost disappears at higher (30 eV) energies. Attack at the reactive halogen end produces the most ions, regardless of the polarity of that end. For each molecule, the reactive end seems to have the lower threshold energy. These observations may be a result of the electron being transferred to a specific end of the molecule, but the experiments measure only the net result of an electron transfer followed by the separation of the ions. Whether or not electron jump per se depends on orientation is still an open question, but the authors are able to qualitatively interpret the experimental results as being due to interactions between the ions as they separate in the exit channel. Most of the negative molecular ions dissociate, ejecting a halogen X⁻ in the direction of the (oriented) molecular axis. If the X end is oriented away from the incoming K atom, the ejected X⁻ will travel in the same direction as the K{sup +}, making the electron more likely to return to the K{sup +} ion and reducing the K{sup +} signal in this unfavorable orientation.
    The Journal of Physical Chemistry. 04/2002; 95(21).
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    ABSTRACT: Theoretical calculations based on the long-range dynamics of the harpoon model have been used to predict the effects of reactant orientation upon ionization yields and angular distributions of product MX for reactions of alkali-metal atoms M with oriented symmetric-top molecules of general formula CYâX, where X is a halogen and Y is H, F, or methyl. Large effects of orientation upon ionization yield near threshold are found to result from differences in the partitioning of kinetic energy between relative motion and center-of-mass motion, depending on whether the X⁻ ion ejected by the unstable CYâX⁻ ion is traveling in the same direction as the incoming M{sup +} ion or the opposite direction. For most systems for which comparison is possible, excellent agreement is obtained with experimental data on the effect of orientation upon ionization yield. For ionizing collisions of fast K atoms with CHâI and CFâI and CFâBr, the theory is less successful and it appears that orientation-dependent short-range interactions must play a major role.
    The Journal of Physical Chemistry 04/2002; 96(4).
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    Howard S. Carman, Peter W. Harland, Philip R. Brooks
    The Journal of Physical Chemistry. 04/2002; 90(5).
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    Sean A. Harris, Philip R. Brooks
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    ABSTRACT: Collisions between neutral K atoms and oriented t-butyl bromide molecules produce the ions K+ and Br− at energies high enough to separate charged particles (≳4 eV). Ions are detected by coincidence tof mass spectrometry for orientation of the t-butyl bromide such that the K atom attacks either the Br end or the t-butyl end of the molecule. At high energies the steric asymmetry factor is larger than that for CH3Br. But at energies near threshold, the steric asymmetry factor reverses sign and attack at the t-butyl end becomes more reactive than attack at the Br end. The electron is apparently transferred into different orbitals at different ends.
    The Journal of Chemical Physics 06/2001; 114(24). · 3.16 Impact Factor
  • Jiping Zhan, Beike Jia, Philip R. Brooks
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    ABSTRACT: Fast atoms collide with molecules spatially oriented by hexapole state selection techniques. Collisions of the neutral species at energies of about 4 eV produce positive and negative ions which are detected by time of flight mass spectrometry. Electron transfer from the atom produces negative ions, and frequently these ions are different from those formed by bombardment with free electrons. Orientation normally has a large effect on electron transfer and in cases where several negative ions are formed, different ions may be preferentially formed at different ends of the molecule.
    10/2000;
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    Sean A. Harris, Peter W. Harland, Philip R. Brooks
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    ABSTRACT: Positive and negative ion pairs are detected in collisions of fast [5–30 eV center of mass (CM)] K atoms with CH3CN molecules oriented in space. Different products are favored by attack at different ends of the molecule. The principal negative ion is CN−, which is produced preferentially upon attack at the negative, or CN-end of the molecule. The cyanomethyl ion, CH2CN−, is observed as the energy is raised, and is preferentially produced upon attack at the positive (methyl) end of the molecule. Formation of these ions appears to proceed by electron transfer into the LUMO at either end of the molecule, followed by a curve crossing into the most exoergic channel. Neither parent ions nor electrons are detected.
    Physical Chemistry Chemical Physics 01/2000; 2(4):787-791. · 3.83 Impact Factor
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    ABSTRACT: The spatial scrambling of upper-Stark-state (KMJ<0)-selected beams of CH3F, CH3Cl, CH3Br, and CH3I in field-free space has been investigated. It has been proposed that the mechanism for spatial deorientation as the electric-field strength is reduced to zero is the change in precessional frequency and loss of spatial direction as the total angular momentum J decouples from the collapsing electric field and couples with the nuclear spin. Supersonic beams were quantum-state selected in a hexapole inhomogeneous electric field and directed between a pair of parallel field plates before being focused through a second hexapole field to a quadrupole mass spectrometer detector. Exposure of the beam to zero field in the parallel-plate region leads to an attenuation of the beam signal relative to the non-zero-field case due to defocusing of newly formed lower Stark states and KM=0 states in the second hexapole field. This phenomenon can be used to determine the effect of field strength on the orientation of upper Stark states within the beam. The beam signal at the detector was shown to remain constant for uniform field strengths greater than approximately 3 V cm-1, with a signal attenuation of around 40% relative to this level at zero field. Attempts were made to measure the mean lifetime for spatial scrambling by pulsing the uniform field to ground potential for increasing intervals and observing the beam attenuation. However, these measurements were complicated by the effect of the beam velocity distribution on the signal and it was found that reproducible values could not be obtained, though the results of these experiments are consistent with lifetimes lying in the expected range from 100 to 300 μs.
    Physical Review A 10/1999; 60(4):3138-3143. · 3.04 Impact Factor
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    ABSTRACT: Molecular-beam electric-resonance spectroscopy is used to interrogate the rotational states present in a molecular beam of oriented symmetric-top molecules produced for scattering experiments. ΔM = ±1 transitions are observed between Stark energy levels in weak electric fields and depend on the rotational quantum numbers J and K. Substantial rotational cooling is apparent in both neat and seeded beams. Each resonance signal has a complicated dependence upon the high voltage applied to the hexapole focusing fields because molecules in the newly transformed states have vastly different focusing properties from the original. These effects can be unified using a “reduced” focusing voltage that allows intensity comparisons between rotational states, giving rotational temperatures of 3−4 K for CF3H seeded in He or Ar. Under favorable circumstances, radio frequency “labeling” might allow one to selectively remove one rotational level at a time from an oriented molecular beam and thereby to study the orientation dependence of different rotational states.
    The Journal of Physical Chemistry A 01/1998; 102(7). · 2.77 Impact Factor
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    The Journal of Physical Chemistry. 01/1996; 100(19):8008-8010.
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    ABSTRACT: The reaction K + NaBr --> KBr + Na is probed during the reactive collision by a continuous wave laser tuned to frequencies not resonant with excitation in either reagents or products. Transient [K..Br..Na] absorbs a laser photon giving [K..Br..Na](*), which can decompose to Na(*) + KBr. Emission from excited Na(*) at the sodium D lines provides direct evidence of laser absorption during the reaction. Different excitation spectra were observed, depending on which sodium D line was monitored. This difference is explicable if, in the absence of the laser, the reaction flux partially bifurcates to a second potential energy surface during the reaction.
    Science 09/1993; 261(5127):1434-6. · 31.03 Impact Factor