Mark W. Crofton

Infrared spectroscopy of carbo‐ions. III. ν3 band of methyl cation CH+3

The infrared spectrum of the degenerate ν3 vibration–rotation band of methyl cation CH+3, one of the most fundamental carbo‐ions, has been observed and analyzed. The spectrum was observed in the frequency range of 3240–2960 cm−1 using the difference frequency laser system as the tunable coherent infrared source. A gas mixture of He:H2:CH4∼700:20:1 with a total pressure of ∼7 Torr was used for the ac discharge in an air‐cooled and a water‐cooled multiple inlet–outlet discharge tube. The velocity modulation method together with noise subtraction and the unidirectional multiple passing of the beam was used for high sensitivity. The spectral lines of CH+3 appeared amid many other lines of C2H+2 and C2H+3 but their widths and their response to chemical conditions gave a good initial clue for the identification. Over 200 absorption lines have been assigned to CH+3 and the isotopic species 13CH+3. The spectral pattern clearly shows that the equilibrium molecular structure is a planar, equilateral triangle as exp...

Infrared spectroscopy of carbo‐ions. V. Classical vs nonclassical structure of protonated acetylene C2H+3

The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air‐cooled and water‐cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen‐cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler‐limited, high sensitivity spectroscopy.The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pat...

Infrared studies of molecular ions. I. The ν3 band of NH4

Infrared spectrum of the ν3 fundamental vibration‐rotation band of the ammonium ion NH4+ has been observed. The ac discharge method recently developed by Saykally and his colleagues was used with a difference frequency laser system as the radiation source. Some 150 lines have been observed and assigned to P,q, and R branches up to J=10. An analysis of the low J transition gave the following molecular constants. ν0−2(Bζ)′ =3342.573 (21) cm−1, B′−(Bζ)′=5.523 53(70) cm−1, B′−B0=−0.049 997(19) cm−1, Zt=−7.59(80)×10−3 cm−1, Ft=−1.6(11)×10−4 cm−1, and Ds=7.2(32)×1p−5 cm−1. A calculated Coriolis constant was used to obtain the ν0 and b2d0 values as ν0=3343.130 (56) cm and b0=5.852 (31) cm−1. These results confirm the expected tetrahedral structure of this molecular ion and give an estimated equilibrium bond length of 1.026(5) A. (AIP)

Infrared spectroscopy of carbo-ions. VI: C-H stretching vibration of the acetylene ion C2H2+ and isotopic species

The infrared spectra of the band of the 2Π–2Π asymmetric hydrogen stretching vibration in the three isotopic acetylene ions C2H2+ (ν3), 13C2H2+ (ν3), and DCCH+ (ν1) have been observed and analyzed. The high resolution infrared spectra were recorded using a difference‐frequency laser spectrometer as the tunable coherent infrared source probing an ac glow discharge. Velocity modulation, noise subtraction, and unidirectional multipassing of the infrared beam through the discharge cell provided high sensitivity. C2H2+ was produced in a gas mixture of H2, He, and either CH4 or C2H2, with a total pressure of ≊7 Torr in multiple‐inlet–outlet air‐, water‐, and liquid‐nitrogen‐cooled discharge tubes; C2H2 freezing precluded its use in liquid‐N2‐cooled discharges. Complicated by a strong perturbation whose maximum occurred at N’=15 for F1 and N’=14 for F2, the assignment of the spectrum of normal C2H2+ was made possible by (1) fortuitous discharge conditions which provided unambiguous discrimination of C2H2+ lines ...

Observation and analysis of the ν2 and ν3 fundamental bands of the H2D+ ion

The high‐resolution absorption spectrum of the D2H+ molecular ion in the 1800–2300 cm−1 region has been measured in a discharge through a mixture of H2 and D2 using a tunable infrared diode laser source and a cooled hollow‐cathode absorption cell. A total of 72 new lines of D2H+ have been observed, as well as five previously measured in ion‐beam experiments by Wing and Shy, and these have been assigned to specific rotational transitions of the ν2 and ν3 fundamental bands. Two different and complementary theoretical models are used to fit these data: one is an A‐reduced asymmetric rotor effective Hamiltonian including the Coriolis and higher‐order rotational interactions between ν2 and ν3; and the other is a supermatrix model in which the matrix of the untransformed Hamiltonian is set up and diagonalized directly, using a large vibration–rotation basis that diagonalizes the vibrational energy. The former approach is less expensive and provides a better fit, but because of the large number of parameters var...

Infrared spectra of carboions. II. ν3 band of acetylene ion C2H+2(2Πu)

The infrared spectrum of the ν3 band of C2H+2(2Πu) has been observed and analyzed. This is the first observation of this fundamental molecular ion in any spectral range.

Infrared spectroscopy of carbo‐ions. IV. The A 2Πu–X 2Σ+g electronic transition of C−2

The infrared spectrum of the A 2Πu←X 2∑+g electronic transition of C−2 has been observed under high resolution and analyzed. Three bands (v’←v)=(0,0), (1,1) and (0,1) have been observed; the first two bands were observed by using the difference laser frequency system in the frequency range of 3960–3780 cm−1 and the last by using a diode laser in the frequency range of 2210–2120 cm−1. A gas mixture of 50 mTorr of acetylene and 7 Torr of He was used for the ac discharge in an air‐cooled and a water‐cooled multiple inlet–outlet discharge tube. The simplicity of the optimum gas mixture suggested that C−2 is produced directly by simple dissociative electron attachment of acetylene. Altogether 103 absorption lines have been observed and accurately measured. Most of them are P, Q, and R form branches of allowed F1↔F1, F2↔F2 transitions although some forbidden F1↔F2 transitions and O and S transitions have also been measured. The hot bands (1,1) and (0,1) have been observed with intensity which is less than that ...

Infrared spectrum of the fundamental vibration–rotation band of OD−

The fundamental v=1←0 vibration–rotation band of OD− has been observed using the tunable infrared radiation from a difference frequency laser system and the velocity modulation technique for detection. The band origin is determined to be 2625.332(3) cm−1. The rotational constant B and the centrifugal distortion constant D have been determined for both the ground state and the first excited state. A remarkable similarity between molecular constants of OD− and OD has been noticed and utilized to estimate equilibrium vibration–rotation constants. These vibration–rotation constants were used to estimate the equilibrium bond length and the quadratic, cubic, and quartic force constants.

Observation of the infrared spectrum of methyl cation CH+3

The vibration‐rotation infrared spectrum of the ν3 fundamental band of the methyl cation CH+3 has been observed using a difference frequency laser system. The analysis of the spectrum demonstrates the D3h symmetry of the ion. This work marks the beginning of high resolution spectroscopy of simple polyatomic hydrocarbon cations.

Infrared spectroscopy of the ν3 band of H2O

Abstract The ν3 band of the H2O+ ion was detected in absorption in the region 3100–3400 cm−1 using a tunable difference frequency spectrometer and the velocity modulation technique for detection. About 70 transitions were measured and analyzed in a two-stage fit in order to obtain information on the rotational constants of both ground and excited vibrational states. Combination differences obtained from our measurements and from the optical spectrum observed earlier by Lew were fit using the A-reduced expression of the Hamiltonian and 16 molecular parameters of the ground state: the rotational constants, the centrifugal distortion constants, and the spin-rotation constants were determined. The band origins of the ν3 band (ν0 = 3253.031(3) cm−1) and of the rovibrationally coupled ν1 band (ν0 = 3213.00(9) cm−1) were determined together with the rotational parameters and the spin-rotation constants of the ν3 band and the vibration-rotation interaction constant.