Infrared bands of 12C2HD

The rotational structure of about 40 bands of ¹²C₂HD observed in the region 6000?600 cm^?1 has been measured and interpreted with the purpose of determining a comprehensive set of molecular constants for this isotopic variety of acetylene. Combining these data with the results for ¹²C₂H₂ and ¹²C₂D₂, a reevaluation of the equilibrium internuclear distances for the acetylene molecule has been made: $\text{r}{}_{\mathrm{e}}\text{(CH) = 1.06215 ± 17 × 10}{}^{\mathrm{?5}}\text{A}\text{?}$ and $\text{r}{}_{\mathrm{e}}\text{(CC) = 1.20257 ± 9 × 10}{}^{\mathrm{?5}}\text{A}\text{?}$ were obtained. This paper presents all the molecular constants derived in this study.     

Microwave optical double resonance and continuous wave dye laser excitation spectroscopy of NO2: Rotational assignment of the K = 0−4 subbands of the 593 nm band

A rotational assignment of approximately 80 lines with K_a′ = 0, 1, 2, 3, and 4 has been made of the 593 nm ${}^2\text{A}{}_1\text{→}{}^2\text{B}{}_2$ band of NO₂ using cw dye laser excitation and microwave optical double-resonance spectroscopy. Rotational constants for the ²B₂ state were obtained as A = 8.52 cm^?1, B = 0.458 cm^?1, and C = 0.388 cm^?1. Spin splittings for the K_a′ = 0 excited state levels fit a simple symmetric top formula and give $\text{(?}{}_{\mathrm{bb}}\text{+ ?}{}_{\mathrm{cc}}\text{)}\text{2}\text{= ?0.0483}\text{cm}{}^{\mathrm{?1}}$. Spin splittings for K_a′ = 1 (N′ even) are irregular and are shown to change sign between N′ = 6 and 8. Assuming that the large inertial defect of 4.66 amu Ų arises solely from A, a structure for the ²B₂ state is obtained which gives $\text{r}\text{(NO) = 1.35}\text{A}\text{?}$ and an ONO angle of 105°. Alternatively, weighting the three rotational constants equally gives $\text{r = 1.29}\text{A}\text{?}$ and θ = 118°.     

The bending vibration bands ν4 and ν5 of HCCI

The bending vibration bands ν₄ and ν₅ of HCCI were studied. From the observed rotational structure the rotational constant B₀ and the centrifugal distortion constant D₀ were obtained. The results were B₀ = 0.105968(7) cm^?1 and D₀ = 1.96(7) × 10^?8 cm^?1 from ν₄ and B₀ = 0.105948(8) cm^?1 and D₀ = 1.96(11) × 10^?8 cm^?1 from ν₅. The structure of the hot bands 2ν₅(Δ) ← ν₅(Π) and 3ν₅(φ) ← 2ν₅(Δ) was also resolved and hence the values α₅ = ?3.033(8) × 10^?4 cm^?1 and q₅ = 9.3(3) × 10^?5 cm^?1 could be derived. The other most intense hot bands following ν₅ could be explained in terms of the Fermi diads $\text{ν}{}_3\text{2ν}{}_5{}^0$ and $\text{ν}{}_3\text{+}\text{ν}{}_5{}^{\mathrm{\pm 1}}\text{3ν}{}_5{}^{\mathrm{\pm 1}}$. Of the numerous hot bands accompanying ν₄, only those between different excited states of ν₄ could be assigned. Then estimates for α₄ and q₄ were also obtained. In addition, several vibrational constants were derived.     

UV-induced double injection in α-sulfur single crystals

The I–V characteristics of the UV-induced double injection of carriers into sulfur single crystals are reported. These characteristics display a quadratic behavior as a function of the electric field up to E≈7×10³$\text{Volts}\text{cm}$ where it changes to a cubic law dependence. The V² branch can be accounted for by means of the one carrier space charge limited current. The cube law dependence arises as a result of the recombination limited two-carrier injection according to the model of Lampert and Mark. We estimate values for the common lifetime τ = 4.8×10^?3 sec, and the trap modulated mobility of holes $\text{μ}{}_{\mathrm{<mtext>h</mtext>}}\text{θ}{}_{\mathrm{<mtext>h</mtext>}}\text{=8.9×10}{}^{\mathrm{?6}}\text{cm}{}^2\text{V sec}$.     

Baroemf in the metal—superionic-metal system: AgAg4RbJ5Ag

It is shown that the application of pressure to one of the electrodes of the AgAg₄RbJ₅Ag system causes a flow of Ag⁺ ions into a region of lower pressure, thus giving rise to baroemf. The value of the baroelectric coefficient is determined by the volume-to-charge ratio of “metallic” ion: $\text{α =}\text{v}{}_0\text{e}\text{= (8?9) × 10}{}^{\mathrm{?11}}\text{V Pa}{}^{\mathrm{?1}}$, which agrees quite well with the measured values of (7–9) × 10^?11 V Pa^?1. Freshly prepared samples exhibit a non-equilibrium enhancement of baroemf, so that α differs somewhat from the above value.     

Microwave spectrum of sulfur difluoride in the first excited vibrational states vibrational potential function and equilibrium structure

Microwave spectra of SF₂ in the first excited states of the three normal modes were observed and analyzed. A comparison of the observed inertia defects in the ν₁ and ν₃ states with those calculated by omitting the contributions of the Coriolis interaction between the two modes led to a $\text{ν}\text{?}{}_1\text{-}\text{ν}\text{?}{}_3$ vibrational frequency differences of 25.72 ± 0.33 cm^?1, with ν₁ being definitely higher. The inertia defect in the ground state and our measured values for the inertia defect in the ν₂ state and for the $\text{ν}\text{?}{}_1\text{-}\text{ν}\text{?}{}_3$ difference were combined with the centrifugal distortion constants of Kirchhoff et al. [J. Mol. Spectrosc.48, 157–164 (1973)] to improve the harmonic force field. The interaction constant between the two SF stretching coordinates was determined precisely. The third-order and the cubic anharmonic potential constants were calculated from the observed vibration-rotation constants. The equilibrium structure was determined to be $\text{r}{}_{\mathrm{e}}\text{(}\text{SF}\text{) = 1.58745 ± 0.00012}\text{A}\text{?}$ and θ_e(FSF) = 98.048 ± 0.013°.     

The quadrupole moments of the 5− states in 116Sn, 118Sn and 120Sn

The quadrupole interaction frequencies $\text{ω}{}_0\text{=}\text{3eQ}{}_1\text{V}{}_{\mathrm{zz}}\text{41(21-1)}\text{h}\text{?}$ in the 5^? state of ¹¹⁸Sn have been measured by time differential perturbed angular correlation technique in Sn, Sb and (95% Sn+5% Sb) environments. The ω₀ for ¹¹⁶Sn was determined in Sn environment only. With the help of the known electric field gradient ¹) of Sn in a Sn lattice the quadrupole moments have been deduced as $\text{Q(5}{}^{\mathrm{?}}\text{,}{}^{118}\text{Sn}\text{) = ±0.10(4) b}\text{and}\text{Q(5}{}^{\mathrm{?}}\text{,}{}^{116}\text{Sn}\text{) = ±0.165(60)}\text{b}$. These values together with the known²) quadrupole moment of the analogous 5^? state in ¹²⁰Sn are interpreted in terms of the pure single-particle model. The data exhibit the expected strong systematic variation of Q_I with the number of particles in the h_{$\text{11}\text{2}$}. subshell which is being filled with 1, 3 and 5 neutrons in ¹¹⁶Sn, ¹¹⁸Sn, and ¹²⁰Sn, respectively.     

Polarization spectroscopy of the E0g+-B0u+ band system of Br2

The E-B (0_g⁺-0_u⁺) band system of Br₂ has been investigated at Doppler-limited resolution using polarization labeling spectroscopy. Merged E state data for the three naturally occurring isotopes in the range v_E = 0–16, expressed in terms of the constants for ⁷⁹Br₂, are (in cm^?1) Y_0,0 = 49 777.962(54), Y_1,0 = 150.834(22), Y_2,0 = ?0.4182(28), Y_3,0 = 6.6(11) × 10^?4, Y_0,1 = 4.1876(28) × 10^?2, Y_1,1 = ?1.607(16) × 10^?4, and Y_0,2 = 1.39(39) × 10^?8. The bond distance is $\text{r}{}_{\mathrm{<mtext>e</mtext>}}\text{= 3.194}\text{A}\text{?}$, and the diabatic dissociation energy to $\text{Br}{}^{\mathrm{+}}\text{(}{}^3\text{P}{}_2\text{) +}\text{Br}{}^{\mathrm{?}}\text{(}{}^1\text{S}{}_0\text{)}\text{is 34 700 cm}{}^{\mathrm{?1}}$.     

Microwave spectrum,structure, dipole moment,quadrupole coupling constant,and internal motion of thioformamide

The r_s structure of thioformamide has been determined from the microwave spectra of the normal as well as isotopic species of the molecule. The structural parameters obtained assuming the planarity of the molecule are $\text{NH}{}_{\mathrm{c}}\text{= 1.001}{}_8\text{± 0.006}\text{A}\text{?}\text{,}\text{NH}{}_{\mathrm{t}}\text{= 1.006}{}_5\text{± 0.003}\text{A}\text{?}\text{,}\text{CN}\text{= 1.358}{}_2\text{± 0.003}\text{A}\text{?}\text{,}\text{CS}\text{= 1.626}{}_2\text{± 0.002}\text{A}\text{?}\text{,}\text{CH}{}_{\mathrm{a}}\text{= 1.09}{}_6\text{± 0.08}\text{A}\text{?}\text{, ?}\text{H}{}_{\mathrm{c}}\text{NH}{}_{\mathrm{t}}\text{, = 121°42′ ± 40′, ?}\text{H}{}_{\mathrm{c}}\text{NC}\text{= 117°55′ ± 40′, ?}\text{H}{}_{\mathrm{t}}\text{NC}\text{= 120°22′ ± 30′, ?}\text{NCS}\text{= 125°16′ ± 15′ ?}\text{NCH}{}_{\mathrm{a}}\text{= 108°}{}_{\mathrm{5\prime }}\text{± 5°,}\text{and}\text{?}\text{SCH}{}_{\mathrm{a}}\text{= 126°}{}_{\mathrm{39\prime }}\text{± 5°}$.The dipole moment is calculated from the Stark effects of the three transitions to be μ_a = 3.99 ± 0.02 D, μ_b = 0.13 ± 0.25 D, and μ_total = 4.01 ± 0.03 D, where the c component is assumed to be zero.The quadrupole coupling constant of the ¹⁴N nucleus is estimated using the doublet splittings observed for six Q-branch transitions; χ_cc - χ_bb = ?5.39 ± 0.15 MHz and χ_aa = 2.9 ± 1.2 MHz.Two sets of vibrational satellites are observed and assigned to the first excited state of the amino wagging and the NCS bending vibrations, respectively. The relative intensity measurement gives the vibrational energies of 393±40 cm^?1 and 457 ± 50 cm^?1 for NH₂CHS and 293 ± 30 cm^?1 and 393 ± 40 cm^?1 for ND₂CHS. The amino wagging inversion vibration in the molecule is discussed in comparison with that in formamide. It is most probable that the thioformamide molecule is also planar without any potential hump to the amino inversion at the planar configuration.     

42P fine-structure mixing in potassium by collisions with N2, H2, CO,and CH4

Cross sections for $\text{4}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?4}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ fine-structure mixing in potassium, induced in collisions with N₂, H₂, CO, and CH₄, were determined by methods of atomic fluorescence spectroscopy. The experiments were carried out at a temperature of 342 K, a K density of 6 × 10⁹ cm^?3 and mtorr buffer gas pressures, and yielded the following cross sections $\text{Q}{}_1\text{(4}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{→4}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$) and $\text{Q}{}_2\text{(4}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{→4}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ (in units of 10^?6 cm²): for N₂, 79 and 54; for H₂, 57 and 39; for CO, 98 and 64; for CH₄, 86 and 58. The values for N₂ and H₂ supersede those reported previously by McGillis and Krause (1968).     

The 2780 Å absorption system of thiophosgene

The vapor phase absorption spectrum of thiophosgene (Cl₂CS) in the 2500–2900 Å region consists of a broad intense band ($\text{log}\text{?}{}_{\mathrm{<mtext>max</mtext>}}\text{= 3.5}\text{at}\text{2540}\text{A}\text{?}$. On the red side of this a vibrationally discrete structure is found which becomes increasingly diffuse and merges into the broad band as the wavelength is decreased. It is shown that this vibrational structure can be explained as due to a $\text{π → π}{}^1$, ${}^1\text{A}{}_1\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ electronic transition between a planar ground state and a pyramidal excited state of the molecule. In the latter state, the CS stretching mode ν₁′(a₁) = 681 cm^?1 and the CCl bending mode ν₃′(a₁) = 147 cm^?1. From the inversion doublet splitting of the out-of-plane mode ν₄′(b₁), the barrier to inversion is calculated to be ～126 cm^?1, with an equilibrium out-of-plane angle of ～20°.     

Temperature and pressure dependence of the elastic property of GeS2 glass

The sound velocities in GeS₂ glass have been measured by means of ultrasonic interferometry as a function of temperature or pressure up to 1.8 kbar. The bulk modulus K_s = 117.6 kbar and shear modulus G = 60.60 kbar were obtained for GeS₂ glass at 15°C and 1 atm. The temperature derivatives of both sound velocities and elastic moduli are negative :

$\text{(}\text{?ν}{}_1\text{?T}\text{)}$

_p =

$\text{?1.54 × 10}{}^{\mathrm{?4}}\text{km}\text{sec}$

°C,

$\text{(}\text{?ν}{}_1\text{?T}\text{)}$

_p =

$\text{?1.27× 10}{}^{\mathrm{?4}}\text{km}\text{sec}$

°C and

$\text{(}\text{?K}{}_{\mathrm{s}}\text{?T}\text{)}$

_p =

$\text{?1.27 × 10}{}^{\mathrm{?2}}\text{kbar}\text{°C}$

,

$\text{(}\text{?G}\text{?T}\text{)}$

_p = ?1.23 × 10^?2 kbar/°C,

$\text{(}\text{?Y}\text{?T}\text{)}$

_p = ?2.93 × 10^?2 their pressure derivatives are positive:

$\text{(}\text{?ν}{}_1\text{?P}\text{)}$

_T = 4.43× 10^?2km/kbar,

$\text{(}\text{?ν}{}_1\text{?P}\text{)}$

_T =

$\text{0.633 × 10}{}^{\mathrm{?2}}\text{km}\text{kbar}$

and (?K_s?P0)_T=6.81,

$\text{(}\text{?G}\text{?P)}{}_{\mathrm{T}}$

= 1.03, (?Y?T_T= 3.57. The Grüneisen parameter, γ_th= 0.298, and the second Grüneisen parameter, δ_s = 3.27, have also been calculated from these data. The elastic behavior of GeS₂ glass has proved to be normal despite the structural similarity among the tetrahedrally coordinated SiO₂, GeO₂ and GeS₂ glasses.     

Infrared diode laser spectroscopy of the CF radical

The fundamental bands of the CF radical in the $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electronic states were observed by using an infrared tunable diode laser as a source. Zeeman modulation could be used in detecting lines not only in the ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ state, but also in ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, because the CF radical deviates considerably from Hund's case (a). From the least-squares analysis of the observed spectra, the following molecular constants were obtained: B_e = 1.416 704 (37) cm^?1, α_e = 0.018 419 (50) cm^?1, $\text{r}{}_{\mathrm{e}}\text{= 1.271 977 (17)}\text{A}\text{?}$, D_e = 6.68 (15) × 10^?6cm^?1, p₀ = 0.008 580 (21) cm^?1, p₁ = 0.008 52 (11) cm^?1, and $\text{ν}{}_0\text{= 1286.1281 (5)}\text{cm}{}^{\mathrm{?1}}$, with three standard errors in parentheses.     

Magnetothermal effect in semiconductors in strong magnetic fields

We have derived an expression for the adiabatic differential temperature change $\text{(}\text{?T}\text{?H}\text{)}{}_{\mathrm{s}}$ of a semiconductor in the extreme quantum region. The transition to non-degeneracy is characterised by a sharp negative dip in $\text{(}\text{?T}\text{?H}\text{)}{}_{\mathrm{s}}$, which may be greatly modified by the Zeeman splitting. A numerical calculation of $\text{(}\text{?T}\text{?H}\text{)}{}_{\mathrm{s}}$ has been made for GaAs with electron concentrations of 1.2 × 10¹⁶ and 3.8 × 10¹⁶ cm^?3 at temperatures of 2,1 and 0.5 K.     

Analysis of the Raman spectrum of SF6

The Raman active fundamentals ν₁(A1g), ν₂(Eg), ν₅(F2g), and the overtone 2ν₆ of SF₆ have been investigated with a higher resolution and the band origins were estimated to be: ν₁ = 774.53 cm^?1, ν₂ = 643.35 cm^?1, ν₅ = 523.5 cm^?1, and 2ν₆ = 693.8 cm^?1. Raman and infrared data have been combined for estimation of several anharmonicity constants. The ν₆ fundamental frequency is calculated as 347.0 cm^?1. From the analysis of the ν₂ Raman band, the following rotational constants of both the ground and upper states have been calculated:

$\text{B}{}_0\text{= 0.09111 ± 0.00005}\text{cm}{}^{\mathrm{?1}}\text{; D}{}_0\text{= (0.16±0.08)10}{}^{\mathrm{?7}}\text{cm}{}^{\mathrm{?1}}$

;

$\text{B}{}_2\text{= 0.09116 ± 0.00005}\text{cm}{}^{\mathrm{?1}}\text{; D}{}_2\text{= (0.18±0.04)10}{}^{\mathrm{?7}}\text{cm}{}^{\mathrm{?1}}$

.     

Microwave spectrum and structure of cyanamide: Semirigid bender treatment

An extensive study of the microwave spectrum of cyanamide has been undertaken, the analysis being based in part on semirigidbender calculations by the methods of Bunker and Szalay. Inversion lines of NH₂CN, $\text{K}{}_{\mathrm{?1}}\text{= 2}{}^{\mathrm{a}}\text{Q}$ branches and a number of vibrational satellites of the J = 2?1 transition were observed. A two-vibrational-state Hamiltonian was used to fit simultaneously the 0⁺ and 0^? microwave data and yielded rotational constants X, Y, Z, D_J, D_JK, d₁, H_JK as well as the inversion splitting and the μ_yz-connecting matrix element. Vibrational satellite data of seven isotopic species and infrared frequencies of NH₂CN were included in the semirigid bender calculations: The NCN spine is nonlinear by ca. 5° in the equilibrium structure of the molecule. Also, $\text{r}{}_{\mathrm{<mtext>NH</mtext>}}\text{A}\text{?}\text{= 0.9994 + 0.0144?}{}^2$; <HNH/2 = 60.39° ? 0.1134?²; $\text{r}{}_{\mathrm{<mtext>NC</mtext>}}\text{A}\text{?}\text{= 1.3301 + 0.0327?}{}^2$ (? is the inversion angle in rad); $\text{r}{}_{\mathrm{<mtext>CN</mtext>}}\text{= 1.1645}\text{A}\text{?}$ fixed. The inclusion of the NC bond flexing was necessary in order to reproduce the observed vibrational satellite patterns of NH₂CN, NHDCN, and ND₂CN. The barrier to inversion of the amino group is 510 ± 6 cm^?1 with minima at ±45.0 ±0.2°. The inversion dipole moment is 0.91 ± 0.02 Debye.     

The spectroscopic study of excitation and ionization in the collision of two metastable atoms

Collisions between two excited atoms leading to an increase in the excitation energy of the particles have been under investigation. All measurements were made in the afterglow of gas-discharge plasma. The cross sections of the following reactions have been determined: $\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) +}\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) →}\text{Hg}{}^7\text{+}\text{Hg}\text{(6}{}^1\text{S}{}_0\text{),}\text{He}{}_{\mathrm{m}}\text{(2}{}^{\mathrm{1,3}}\text{S}\text{) +}\text{Xe}{}_{\mathrm{m}}\text{(}{}^3\text{P}{}_{\mathrm{0,2}}\text{) → (}\text{Xe}{}^{\mathrm{+}}\text{)}{}^1\text{+}\text{He}{}_0\text{+}\text{e}$. The cross section of the first reaction for different transitions lies in the region (2?35) × 10^?15 cm² and the cross section of the second, in (0.2?2.4) × 10^?16 cm². Possible systematic errors and the role of cascade transitions are discussed. Cross sections of the Penning reaction He_m + Xe₀ → He₀ + Xe⁺ + e have also been measured. The result is σ (2³S) = (1.4 ± 0.2) × 10^?15 cm², σ (2¹S) = (1.2 ± 0.3) × 10^?15 cm².     

The reduction of NO with D2 over the (110) surface of iridium

The heterogeneously catalyzed reaction between NO and D₂ to produce N₂, ND₃ and D₂O over Ir(110) was investigated under ultra-high vacuum conditions for partial pressures of the reactants between 5 × 10^?8 and 1 × 10^?6Torr, total pressures between 10^?7 and 10^?6 Torr, and surface temperatures between 300 and 1000 K. Mass spectrometry, LEED, UPS, XPS and AES measurements were used to study this reacting system. In addition, the competitive coadsorption of NO and deuterium was investigated via thermal desorption mass spectrometry and contact potential difference measurements to gain further insight into the observed steady state rates of reaction. Depending on the ratio of partial pressures ($\text{R }\text{P}{}_{\mathrm{<mtext>D</mtext>}}{}_2\text{P}{}_{\mathrm{<mtext>NO</mtext>}}$), the rate of reduction of NO to N₂ shows a pronounced enhancement when the surface is heated above a critical temperature. As the surface is cooled, the rate maintains a high value independent of temperature until a lower critical temperature is reached, where the rate drops precipitously. This hysteresis is due to a change in the structure and composition of the surface. For sufficiently large values of R and for an “activated” surface, N₂ and ND₃ are produced competitively between 470 and 630 K. Empirical models of the different regimes of the steady state reaction are presented with interpretations of these models.     

Observation of muon pairs in neutrino interactions at Serpukhov 70 GeV accelarator

A search for dimuons produced in a spark chamber experiment in neutrino and antineutrino beams of the Serpukhov accelerator is reported. The clear dimuon signal has been observed in vN interactions. R_v = N(2ω)/N(1ω)? (6.2 ± 1.7) × 10^?3 in the energy interval 7.5 ÷ 30 GeV. From antineutrino data we conclude that in the same energy range $\text{R}{}_{\mathrm{<mtext>v</mtext>}}\text{? 1.1. × 10}{}^{\mathrm{?2}}\text{(90\% C.L.)}$.     

Oscillator strengths in the Cameron system of carbon monoxide

Relative oscillator strengths in the Cameron system of CO(a³Π ← X¹Σ) have been observed in absorption for six bands (υ′ = 0–5, υ″ = 0) with the result, normalized to the absolute (0, 0) band measurement of Hasson and Nicholls, $\text{?}{}_{00}\text{= (1.62±0.07) × 10}{}^{\mathrm{?7}}$, $\text{?}{}_{10}\text{= (1.96±0.09) × 10}{}^{\mathrm{?7}}$, $\text{?}{}_{20}\text{= (1.41±0.04) × 10}{}^{\mathrm{?7}}$, $\text{?}{}_{\mathrm{3\; 0}}\text{= (0.72±0.03) × 10}{}^{\mathrm{?7}}$, $\text{?}{}_{40}\text{= (0.31±0.02) × 10}{}^{\mathrm{?7}}$, $\text{?}{}_{50}\text{= (0.14±0.01) × 10}{}^{\mathrm{?7}}$. The density of CO was modulated with a motor-driven vacuum valve and synchronous fluctuations (?1 per cent) in the transmitted intensity detected with a lock-in amplifier. Peak pressure in the 21 cm absorption cell was approximately 10 torr. A curve of growth analysis was used to correct saturation effects by less than 3 per cent.