R. Marx

Radiative relaxation of NO+ (X, v = 1−4)

Abstract Radiative lifetimes of NO + (X, v =1–4) has been determined using the monitor ion technique in a triple cell ICR spectrometer with Fourier transform detection. Correction of the measured lifetimes for radiative cascades and quenching is discussed in detail. The corrected lifetimes are in very good agreement with the best available theoretical data.

Radiative relaxation of vibrationally excited ions

Abstract A triple cell ICR spectrometer analogous to a three-stage mass spectrometer (MS/MS/MS) but capable of studying thermal energy collisions is described. The facility to store ions in a good vacuum allows the measurement of radiative lifetimes using a chemical reaction to probe the energy contents of the ions as a function of their storage time. Vibrational lifetimes have been measured for two diatomic ions in their electronic ground state: NO+ and HCl+. NO+ and NO have similar lifetimes, in very good agreement with calculated values. HCl+ (v = 1) has a lifetime ten times shorter than HCl (3 ± 1 ms compared with 29 ms), in fairly good agreement with the calculated value of 4.6 ms. This is the first experimental confirmation of the very short lifetimes predicted theoretically for diatomic hydride ions. Qualitative results have been obtained for the radiative lifetime of NH3+. The shape of the decay curves indicates the presence of long-lived (more than 300 ms) and short-lived (10–20 ms) states.

Experimental investigation of vibrational radiative lifetimes: H2O+ and D2O+ ions in their ground electronic state (X 2B1)

Radiative lifetimes of vibrationally excited H2O+ and D2O+ ions in their ground electronic state (X 2B1) have been determined using the monitor ion technique in a triple cell ion cyclotron resonance spectrometer with Fourier transform detection. The monitor reactions are proton or deuteron transfer from H(D)2O+ to CO2 and N2O. The lifetimes are corrected for collisional deactivation and reactions with the background gases occurring during the relaxation time of the ions. N2O probes all the excited vibrational levels of H2O+ and D2O+. For H2O+ only the bending modes (0,v≥1,0) contribute to the decay curve. The corresponding overall lifetime, 26.8±3 ms, is in very good agreement with the computer simulated overall lifetime including the theoretical lifetimes of Weis et al. [J. Chem. Phys. 91, 2818 (1989)] and estimated populations of the bending vibrational levels. For D2O+, the overall lifetime of the (0,v≥1,0) bending modes, 99.5±15 ms, and the lifetime of the (1,0,0) stretching mode, 27.5±4.5 ms, are obs...

Radiative lifetimes and reactivity of metastable NO+(a 3Σ+,v) and O+2(a 4Πu,v)

The radiative lifetimes and reactivity of metastable NO+(a 3Σ+) and O+2(a 4Πu) have been investigated using a triple cell ion cyclotron resonance (ICR) spectrometer. Radiative lifetimes of (100±20) ms and (135±25) ms have been found for NO+(a 3Σ+) v≥1 and v≥0 respectively, with Ar and CO2 as monitor gases. The overall reaction rate of NO+(a 3Σ+,v≥1) with Ar (6.5×10−10 cm3 s−1 ) is 1 order of magnitude larger than the rate of v=0 determined in flowing afterglows. CO2 reacts at collision rate. For O+2 the overall lifetime of v≥0, determined with Ar as monitor, is (30±6) ms in between the two values (7 and 130 ms) found by Bustamente et al. using the photodissociation technique. The possible reasons for the longer lifetimes determined in previous ICR experiments is discussed. In the absence of any ab initio calculation, the experimental results on NO+(a 3Σ+) may only be compared to values calculated using an approximated perturbation model. Unfortunately, the reliability of this model, also used in the previous studies, is very limited because of the large uncertainty on the coupling between the metastable a 3Σ+ and the perturbing radiative A 1Π states.The radiative lifetimes and reactivity of metastable NO+(a 3Σ+) and O+2(a 4Πu) have been investigated using a triple cell ion cyclotron resonance (ICR) spectrometer. Radiative lifetimes of (100±20) ms and (135±25) ms have been found for NO+(a 3Σ+) v≥1 and v≥0 respectively, with Ar and CO2 as monitor gases. The overall reaction rate of NO+(a 3Σ+,v≥1) with Ar (6.5×10−10 cm3 s−1 ) is 1 order of magnitude larger than the rate of v=0 determined in flowing afterglows. CO2 reacts at collision rate. For O+2 the overall lifetime of v≥0, determined with Ar as monitor, is (30±6) ms in between the two values (7 and 130 ms) found by Bustamente et al. using the photodissociation technique. The possible reasons for the longer lifetimes determined in previous ICR experiments is discussed. In the absence of any ab initio calculation, the experimental results on NO+(a 3Σ+) may only be compared to values calculated using an approximated perturbation model. Unfortunately, the reliability of this model, also used in the previ...

Radiative lifetimes for an ion of astrophysical interest: HCO+

Abstract Radiative lifetimes for vibrationally excited ground state HCO+ ions have been determined in a triple-cell FT-ICR spectrometer using the proton transfer reactions with HBr, N2O, HCl, CH4, CO2 and NO to probe the time evolution of HCO+ internal energy. With HBr, N2O, HCl and CH4 two sets of lifetimes are obtained; the long ones (359-50 ms) are assigned to the bending mode levels, and the short ones (10–30 ms) to the CH and CO stretching modes. Correction of the observed lifetimes for collisional quenching is discussed in detail. A simple model has been used to simulate the influence of the radiative cascade. The experimental lifetimes obtained for the bending mode levels are in good agreement with the simulated lifetimes. For the fast components the experimental uncertainty is too large to allow any conclusion to be drawn about the lifetime of the individual stretching modes.

Successive reactions of iron carbonyl cations with methanol

Abstract The reactivity of Fe(CO) n + cations with methanol has been investigated using a triple-cell Fourier transform ion cyclotron resonance apparatus. Successive substitutions of the CO ligands by methanol occur rapidly with similar rates. For n = 3–5 the last CO ligand is not replaced, and the terminal substitution products are FeCO(CH 3 OH) n −1 + . A second reaction pathway is observed for n = 3–5, in which two CO ligands are replaced by one methanol molecule. This pathway occurs even if the Fe(CO) n + ions have been radiatively relaxed before reaction, providing information on the bond energies in the product ions. Strong internal energy effects are observed on the reaction rates and branching ratios for further substitution of the intermediate products. The substitution product Fe(CH 3 OH) + from FeCO + reacts further, with C–O bond cleavage followed by substitution. The resulting Fe[C 2 , H 7 , O 2 ] + ion undergoes either isomerization or collisional deactivation, then slow association with methanol. The reaction of Fe + , due to excited states only, begins with formation of FeOH + and involves the same final steps as in the preceding case.

Radiative lifetimes of Xe+ and Kr+ in their 2P12 spin—orbit states

Abstract Transition probabilities of electric dipole-forbidden transitions 2 P 1 2 → 2 P 3 2 for Kr + and Xe + have been determined using the monitor ion technique in a triple ICR spectrometer with Fourier transform detection. The experimentally determined lifetimes (48.7 ± 5 ms) for Xe + and (340 +70 −50 ms) for Kr + are in very good agreement with theoretical values.

Excitation of C60+ by electrons in a Fourier transform ion cyclotron resonance ion trap: an experimental determination of the radiative cooling rate through its competition with C2 evaporation

Abstract C 60 + ions are produced in an ion trap by crossing a collimated effusive beam of C 60 and an electron beam of energy lower than the C 58 + fragment appearance energy. Being trapped, the C 60 + ions can be further excited by another electron impact, inducing the well-known C 2 evaporation. By varying the electron pulse length while counterbalancing it with the intensity in order to maintain a constant total number of electrons in a pulse, the mean time between two successive electron impacts can be modified. The influence of this time on the amount of fragmentation is direct experimental evidence of the competition between radiative cooling and dissociation, invoked to explain the abnormally high internal energies needed to observe fragmentation even at long times. For ions with internal energies in the range 15.4–22.4 eV, a mean radiative rate constant of 3.3 × 10 2 s −1 has been determined.

Radiative lifetime of CO+ (X2∑+, ν) ions

Abstract The radiative relaxation of CO+(X2∑+, ν) ions produced by ionization of CO and of OCS with 70 eV electrons has been investigated using the monitor ion technique in a triple FT ICR spectrometer. Since the monitor reaction is exothermic for ν ≥ 1, the experimental lifetimes, 170 ms for CO+ from CO and 210 ms for CO+ from OCS, should correspond to the overall decay rate of ν ≥ 2 with a small contribution of ν = 1. These values being much larger than the theoretical lifetime of ν = 1 (128.5 ms), computer-simulated overall lifetimes for ν ≥ 1 and ν ≥ 2 have been determined using theoretical lifetimes calculated by Rosmus and Werner and a vibrational population deduced from photoelectron spectroscopy of CO with 40.8 eV photons (Gardner and Samson). The computer-simulated lifetimes, 250 ms for ν ≥ 1 and 125 ms for ν ≥ 2, are indeed about twice as large as the theoretical lifetimes for ν = 1 and ν = 2, respectively, indicating that the large experimental lifetimes may be accounted for by radiative cascade from the upper vibrational levels.

Radiative lifetimes of HBr+ and DBr+ (2Π1/2, v=0, 1)

Radiative lifetimes of HBr+ and DBr+(2Π1/2, v=0, 1) have been determined using the monitor ion technique in a triple cell ICR spectrometer with Fourier transform detection. The experimental lifetimes corresponding to the vibrational transition v=1→0 are 10.2±1 ms and 56+12−11 ms for HBr+ and DBr+, respectively. The lifetimes calculated by a coupled electron pair approximation (CEPA) method similar to the method used previously for HF+ and HCl+ are 8.8 ms for HBr+ and 35.8 ms for DBr+, in rather good agreement with the experimental results. A comparison with previous experimental and theoretical results on hydrogen halides shows a shortening of the lifetime between the neutrals and the corresponding ions and a lengthening with isotopic substitution (H by D) as well as with substitution of F by Cl or by Br. The lifetimes corresponding to spin–orbit relaxation are very long 852+621−282 ms for HBr+ and 965+546−323 ms for DBr+. They can be considered as equal within the quite large experimental uncertainty on ...