Index of content:
Volume 100, Issue 9, 01 May 1994

Spectroscopic detection and characterization of the FS2 free radical
View Description Hide DescriptionAn extensive vibronic band system of the FS2 radical in the 700–485 nm region has been observed for the first time by laser‐induced fluorescence (LIF) probing of the products of the reaction of F2 with various sulfur‐containing compounds. Jet‐cooled spectra were obtained by reacting fluorine with COS, H2S, or CS2 in the body of a continuous supersonic jet, just prior to expansion. Vibrational analysis of the jet spectra gives ν1 ^{’} = 768, ν2 ^{’} = 217, ν3 ^{’} = 495, ν1 ^{‘} = 705, ν2 ^{‘} = 293, ν3 ^{‘} = 684, and T 00=14 922 cm^{−1}, all in good agreement with ab initio predictions for the FS2 free radical. A rotational analysis of the high‐resolution spectrum of the 3^{5} 0 band shows that the transition is polarized out‐of‐plane, with spin splittings characteristic of a molecule with a single unpaired electron. The rotational constants and spin–rotation interaction constants were determined with good precision for the ground and excited states. By combining the rotational constants with our ab initio estimates of the S–F bond length, the following structural parameters were obtained: r(S–F)=1.651 Å, r(S–S)=1.865(5) Å, and θ(FSS)=109.1(1)° for the X̃ ^{2} A‘ state and r(S–F)=1.642 Å, r(S–S)=2.105(5) Å, and θ(FSS)=97.6(1)° for the Ã ^{2} A’ state.

Two‐color study of Autler–Townes doublet splitting and ac Stark shift in multiphoton ionization spectra of CO
View Description Hide DescriptionBy using a two‐color scheme, we have given a closer scrutiny to the Autler–Townes doublet splitting and ac Stark shift in the 2+2‐photon ionization spectra of CO Ion A ^{1}(v’=4)←X ^{1}Σ^{+}(v‘=0) transition in the presence of another strong laser field, which is resonant with the transition C ^{1}Σ^{+}(v’=0)←A ^{1}Π(v’=4)P 10 branch. All the observations of ac Stark shift/broadening as a function of the perturbing light frequency and its spatial homogeneity strongly support our previous theoretical interpretation for one‐color studies. A qualitative interpretation based on the dressed state theory, which is being quantified, for the Autler–Townes splitting as well as the ac Stark effect has been given. In particular, the magnitude of the Autler–Townes splitting is found to vary with the square root of the perturbing light intensity, in line with the dressed state theory.

Measurements of the microwave spectrum and structural parameters for benzene chromium tricarbonyl
View Description Hide DescriptionMicrowave spectra for four isotopomers of benzene chromium tricarbonyl were measured in the 4–17 GHz range using a Flygare–Balle type pulsed beam spectrometer. Rotational constants obtained are B(^{52}Cr)=732.8886(6) MHz and B(^{53}Cr)=732.8966(3) MHz. Asymmetric top spectra were observed for a single ^{13}C substitution on the benzene ring giving B(^{13}C–bz)=729.9606(3) and C(^{13}C–bz)=727.9024(2) MHz. For a single ^{13}C substitution on one of the carbonyl carbons B(^{13}CO)=731.9036(8) and C(^{13}CO)=729.1657(8) MHz. Since no effects of possible internal rotation were observed on the ^{13}C asymmetric top spectra, we can place a lower limit on the V 6 potential for internal rotation of V 6≳4.0 THz (=1.6 kJ/mole). The centrifugal distortion constants are small, D J =0.05 kHz and D JK =−0.05 kHz, indicating a fairly rigid structure. The ^{53}Cr quadrupole coupling strength is low, eqQ(^{53}Cr)=−12.11(1) MHz, indicating a near octahedral charge distribution around the Cr atom. Structural parameters obtained are the center of the benzene chromium distance r(Cr–bz)=1.67(2) Å, the chromium–carbonyl bond length r(Cr–CO)=1.86(1) Å and the OC–Cr–CO interbond angle α=88(1)°.

Multiphoton studies of jet‐cooled Xe_{2} near the Xe*5d[5/2]^{0} _{3} state: Characterization of the ground and excited state potential curves
View Description Hide DescriptionThe two‐photon resonant, three‐photon (2+1) ionization spectra of jet‐cooled ^{ m }Xe^{ n }Xe, at energies near the Xe* 5d[5/2]^{0} 3 state, are reported. A new progression has been observed and is attributed to transitions from the van der Waals ground state, X ^{1}Σ^{+} g (0^{+} g ), through bound vibrational levels of an excited state of gerade symmetry. The analysis of some 26 closely spaced vibronic bands and isotope effects provides information on the excited and ground state potential energy curves. The vibrational quantum number of the lowest frequency band near 82 539.1 cm^{−1} is assigned to v’=6±1. For v’=6 this leads to molecular constants T e ^{’} ≂ 82 514.9 cm^{−1}, ω e ^{’} ≂ 5.7955 cm^{−1}, and ω ex e ^{’} ≂ 0.07491 cm^{−1}. The upper state can be described by a Morse potential with dissociation energy D e ^{’} ≂ 112.10 ± 0.05 cm^{−1} and internuclear separation R e ^{’} ≂ 5.51 ± 0.03 Å. This is consistent with assignment to a Rydberg molecular state of either the B ^{2}Π1/2g or D ^{2}Σ^{+} 1/2g ion core. At the Xe ^{1} S 0+Xe* 5d[5/2]^{0} 3 threshold the molecular spectrum terminates and continuum absorption is evidenced by a rise and fall in the fragment ion yield. The direct determination of the dissociation limit for the excited state is used to derive the ground state dissociation energy D e ^{‘}≂ 196.32 ± 0.05 cm^{−1}.

Broadband axialization in an ion cyclotron resonance ion trap
View Description Hide DescriptionA novel broadband ion axialization method has been developed for Fourier transform ion cyclotron resonance mass spectrometry based on Penning trapping of ions. Ions of arbitrary mass‐to‐charge ratio range(s) may be driven to the center of the trap (axialization) by azimuthal quadrupolar irradiation with repeated low‐amplitude stored‐wave‐form inverse Fourier transform excitations in the presence of a buffer gas. We demonstrate highly mass‐selective axialization and subsequent high‐resolution Fourier transform ion cyclotron resonance detection of ions spanning a mass‐to‐charge ratio range of 500.

First observation and electronic structure of the diatomic platinum nitride molecule
View Description Hide DescriptionThe optical spectrum of the PtN radical has been observed directly for the first time. The strongest band system [(0,0) bandhead at 18 591 cm^{−1}] displays an unusual ‘‘perpendicular ΔΩ=0’’ perturbation which gives rise to strong intensity cancellation effects. Although a ^{4}Σ^{−} ground state was expected for PtN, we have assigned the ground state as ^{2}Π r ; a ^{4}Σ^{−} assignment is not consistent with our observations. We have used arguments based on both molecular orbital theory and atomic‐ion‐in‐molecule theory to explain the observed ground state symmetry in light of the observed hyperfine structure. Four electronic bands involving a total of six Hund’s case (c) ‖v,Ω〉 substrates have been rotationally analyzed, and many more bands have been observed at lower resolution. The ground state was found to have Ω=0.5 and its principal constants are B e =0.455 708(5) cm^{−1}, α e =0.003 448 1(1) cm^{−1}, spin–rotation constant γ0=0.061 26(7) cm^{−1}, ω e =947.0(5) cm^{−1}, and ω ex e =5.0(5) cm^{−1}. Two other Ω states were observed at low energies—an Ω=1.5 state at T e =2 985.665(2) cm^{−1}, with B 0=0.445 233(8) cm^{−1}, q=1.522(5)×10^{−6} cm^{−1}, ω e =901(1) cm^{−1}, and ω ex e =8(1) cm^{−1}, and another Ω=0.5 state with T e undetermined and other constants remarkably similar to those of the ground state (B 0=0.453 07(6) cm^{−1}, γ0=0.062 02(4) cm^{−1}, ω e =947(1) cm^{−1}, and ω ex e =5(1) cm^{−1}).

Laser induced fluorescence spectroscopy of the B’ 1_{ u }–X 0^{+} _{ g } transition of Xe_{2}: Determination of the B’ state potential and evidence for a barrier to dissociation
View Description Hide DescriptionWe report the observation of discrete and continuous laser induced fluorescence(LIF)spectra of the B’ 1_{ u }–X 0^{+} _{ g } transition of Xe_{2}, near 68 000 cm^{−1}. The discrete features continue 5 cm^{−1} above the predicted atomic asymptote, which indicates the presence of a barrier to dissociation in the excited state. The dissociation energy (D _{ e } ^{’}=48±12 cm^{−1}), and excited state constants (r _{ e } ^{’}=5.46±0.05 Å, ω_{ e } ^{’}=5.9±0.7 cm^{−1}, and ω_{ ex } _{ e } ^{’}=0.17±0.02 cm^{−1}) for the B’ state were obtained from a Franck–Condon fit to the spectrum. The resulting potential is more shallow and has a longer equilibrium bond length compared with a previous experimentally derived potential. The barrier to dissociation (2 cm^{−1}≤h≤10 cm^{−1}, r≊10 Å) is attributed to the presence of a long‐range (∝1/r ^{3}) repulsion, arising from a dipole–dipole resonant interaction.

Real‐time probing of two‐photon absorption with phase related pulses
View Description Hide DescriptionReal‐time probing of two‐photon absorption is formulated. A two‐photon polarizability is introduced and its linear response is derived. The impulse response is shown to depend on the optical phase of incident radiation, and the nonstationary polarizability can be monitored by using a phase‐related pair of pulses. Coherent molecular vibration in ‘‘dark’’ electronic states (accessible only through two‐photon absorption) can be observed. The results are compared to time‐domain observations of coherent vibrations initiated through one‐photon absorption or impulsive stimulated Raman scattering. An estimate of the absolute signal intensity is presented.

Diode laser spectroscopy and coupled analysis of the ν_{2} and ν_{4} fundamental bands of SiH^{+} _{3}
View Description Hide DescriptionThe gas phase infrared spectra of the ν_{2} and ν_{4} fundamentals of SiH^{+} _{3} have been measured between 730 and 1015 cm^{−1}. The ion was produced in an ac glow discharge in silane and hydrogen mixtures. Vibration‐rotation transitions were detected using diode laser velocity modulation spectroscopy. 112 transitions were included in a combined fit of both bands which yielded B _{0}=5.214 51(27) and C _{0}=2.585 20(36) cm^{−1}. The ν_{2} [838.0669(24) cm^{−1}] and ν_{4} [938.3969(36) cm^{−1}] bands are coupled by a Coriolisx,y resonance for which ξ_{24}=−3.8339(22) based on a calculated value of ξ_{4}=−0.051 27. Experimental band origins and rotational and quartic distortion parameters are compared with recent ab initio calculations.

Efficient calculation of highly excited vibrational energy levels of floppy molecules: The band origins of H^{+} _{3} up to 35 000 cm^{−1}
View Description Hide DescriptionRecent testing of a discrete variable representation (DVR) Lanczos product‐basis method to calculate polyatomic vibrational energy levels [M. J. Bramley and T. Carrington, J. Chem. Phys. 99, 8519 (1993)] suggested that, for increasingly floppy molecules, its efficiency will be increasingly competitive with that of contracted‐basis explicit‐diagonalization methods if one can overcome the problem of poor Lanczos convergence caused by kinetic energy singularities. This may be accomplished through the realization that nondirect product finite basis representations (FBRs) (and the related DVRs) can be used efficiently in dynamics calculations for which the rate‐determining step is the evaluation of Hamiltonian matrix–vector products, as is the case with Lanczos recursion [J. W. Tromp and G. C. Corey, J. Chem. Phys. (to be submitted); D. Lemoine and G. C. Corey, J. Chem. Phys. (to be published)]. A synthesis of these two procedures provides a near‐optimally efficient variational vibrational method for molecules for which good basis contraction schemes cannot be designed, and for which the inevitable coordinate singularities require ideally a nondirect product basis. To substantiate this claim, we have performed hybrid DVR/FBR Lanczos calculations of vibrational energies of the classic floppy triatomic molecule H^{+} _{3} up to near dissociation with unprecedently good convergence and unprecedently low computational cost.

Polydiacetylene chains diluted in their single‐crystal monomer matrix
View Description Hide DescriptionThe spectroscopicproperties of the chains of the polydiacetylene (PDA) poly‐4BCMU, diluted in their single‐crystal monomer matrix are studied and discussed in terms of the single polymer chain electronic properties, the monomerstructural changes, and their mutual influence. The polymerexciton transition energy shifts considerably (by 0.3 eV) as temperature decreases, down to a value that may be the lowest transition energy yet observed in a PDA. Two first‐order structural transitions of the monomer crystal are observed near 320 and 220 K. They show up in the polymerspectroscopic properties—absorption and resonance Raman scattering—as discontinuities of both wave numbers and linewidths. So, the polymer chains behave as probes of the monomer crystal structural changes. A new electronic transition is observed below the main exciton one, possibly corresponding to a state of g symmetry. A weak fluorescence is observed at low temperature. Its origin coincides with the exciton absorption. Therefore, there is no configurational relaxation in the exciton state during the fluorescence lifetime.

Phenomenological microscopic treatment of optical activity in molecular crystals
View Description Hide DescriptionExpressions are derived for the optical activity of a molecular crystal showing the contributions arising from molecular chirality and lattice chirality separately and in combination. The molecular chirality, expressed via a generalized field‐gradient polarizability, modifies the effective molecular polarizability in the crystal environment. This modification implies different Davydov splittings in the optical spectra of crystals of enantiomeric molecules that adopt the same chiral structure.

The molecular Stark effect in regions of high state density: Overall simplicity and underlying complexity in the response to a static electric field
View Description Hide DescriptionWe present the high‐resolution (11 MHz) infrared measurement of the molecular Stark effect for the R(0) transition of the acetylenic C–H stretch in 2‐propyn‐1‐ol. The field‐free spectrum is fragmented into three eigenstate components due to the effects of intramolecular vibrational energy redistribution (IVR). As the field strength increases from 0 to 25 kV/cm, the number of eigenstates increases linearly. The center‐of‐gravity of the fragmented R(0) transition follows the simple, second‐order Stark shift (Δν∝E ^{2}) expected for the bright state. However, when viewed at the eigenstate level, the mechanism of the Stark shift is rather complex. At lower field strengths, the eigenstates shift in energy, as occurs for Stark effects in lower state density regimes. As the number of coupled states increases, energy shifting of the eigenvalues is quenched. To preserve the second‐order Stark shift of the center‐of‐gravity, the intensity ‘‘rolls over’’ the largely rigid eigenvalue structure. For molecules in regions of high state density, the reduced energy shifting of the eigenvalues as the electric field is increased means that lack of deflection by inhomogeneous electric fields is not necessarily a consequence of the molecule being nonpolar.

Rotational analysis of the SiD2 Ã ^{1} B 1–X̃ ^{1} A 1 transition observed in a jet
View Description Hide DescriptionThe SiD2 radical was produced by ArF laser photolysis of C6H5SiD3 in a free‐jet expansion, and the laser‐induced fluorescence (LIF) excitation spectrum of the Ã ^{1} B 1–X̃ ^{1} A 1 transition of SiD2 was measured. The LIF excitation spectra of the five vibronic bands, (0,v 2 ^{’},0)–(0,v 2 ^{‘},0), v 2 ^{’}–v 2 ^{‘}= 0–0, 1–0, 2–0, 1–1, and 2–2, were obtained using a narrow‐band dye laser with an intracavity étalon, the resolution of which attained to ∼0.03 cm^{−1}. The rotational structures of the vibronic bands were well analyzed by a Hamiltonian including fourth‐order terms, and the molecular constants were determined for the vibronic levels, v 2=0, 1, and 2, of the Ã ^{1} B 1 and X̃ ^{1} A 1 states. By comparing the observed rotational line intensities with simulated ones, we found two kinds of intensity anomalies depending on the rotational quantum numbers J and K a . We conclude that both the anomalies are caused by a predissociation process to the dissociation continuum, Si(^{3} P)+D2, which was proposed in our previous paper [J. Chem Phys. 96, 44 (1992)]. The K a dependent anomaly was explained by the interaction terms in the Fermi Golden Rule expression for the predissociation process, and the J dependence was interpreted by the final‐state density.

Calculation of vibrational fundamental and overtone band intensities of H_{2}O
View Description Hide DescriptionVibrational intensities are calculated for the fundamental and overtone transitions of H2O up to approximately 18 000 cm^{−1}. The intensities are determined from a dipole moment function expanded in the three internal bond coordinates. The expansion coefficients are computed ab initio at the second‐order Mo/ller–Plesset level of theory with a 6‐311G** basis set. Vibrational wave functions are calculated either from a three‐dimensional harmonically coupled anharmonic oscillator (HCAO) model which uses Morse oscillators to represent both the stretches and the bend of H2O, or from a variational calculation employing the best available potential energy surface and an exact kinetic energy operator. To obtain the most meaningful vibrational intensities we define dipole moment components using the Eckart embedding. Both the HCAO and the variational intensities agree quite well with the experimental results, which span eight orders of magnitude. From the calculations we predict that it may be possible to detect as yet unobserved vibrational transitions of H2O.

Hyperfine structure in high spin multiplicity electronic states: Analysis of the B ^{4}Π–X ^{4}Σ^{−} transition of gaseous NbO
View Description Hide DescriptionThe (0,0) band of the B ^{4}Π–X ^{4}Σ^{−} transition of NbO, near 6600 Å, has been analyzed from spectra taken at sub‐Doppler resolution. The transition is notable for the great width of its Nb nuclear hyperfine structure, which is caused principally by the unpaired 5sσ electron in the ground stateinteracting with the large magnetic moment of the _{41} ^{93}Nb nucleus (I=9/2). A fit to the ground‐state combination differences, including four very precise microwave lines measured by Suenram et al. [J. Mol. Spectrosc. 148, 114 (1991)], has given a comprehensive set of rotational, spin, and hyperfine parameters. Prominent among these are the third‐order spin–orbit distortions of the spin‐rotation interaction and the Fermi contact interaction, which are large and well determined, reflecting different degrees of spin–orbit contamination of the the ^{4}Σ_{1/2} ^{−} and ^{4}Σ_{3/2} ^{−} components of the ground state.
The δ ^{2}π B ^{4}Π state was hard to fit, for a number of reasons. First, its spin–orbit structure is asymmetric, because of strong perturbations by a ^{2}Π state which has been identified in this work, from among the various weak bands in the NbO spectrum near 7000 Å; the result is that many high order centrifugal distortion terms are needed in an effective Hamiltonian model for the rotation. Second, the hyperfine structure is perturbed, not only by this ^{2}Π state, but by distant Σ and Δ states at higher energy. The δ ^{2}σ* C ^{4}Σ^{−} state at 21 350 cm^{−1} appears to be one of these. The distant states generate large apparent nuclear spin‐rotation interactions, both within and between the Λ components of the Π state, as a result of cross terms between matrix elements of the operators −2B J⋅L and a I⋅L. Similar cross terms arising from the operators A L⋅S and a I⋅L produce corrections to the Fermi contact matrix elements and are responsible for the unexpected negative sign of the magnetic hyperfine parameter d. The ‘‘off‐diagonal’’ quadrupole parameter e ^{2} Qq _{2} is very large, and causes some of the higher J line shapes of the B–X system to be noticeably asymmetric at Doppler limited resolution; its value is consistent with the electron configuration of the B ^{4}Π state being δ ^{2}π.

Near infrared 3ν_{2} overtone band of H^{+} _{3}
View Description Hide DescriptionNear infrared spectrum of the 3ν2 overtone band (ν2=3←0) of H^{+} 3 has been observed at 1.4 μm. The spectrum is weaker than the ν2 fundamental band by a factor of ∼250. High sensitivity plasma spectroscopy using velocity modulation and unidirectional multiple passing has enabled us to observe 15 rovibrational transitions. Short‐external‐cavity InGaAsP diodes were used as tunable near infrared radiation sources. The narrow tuning range and the availability of diodes limited the observation to a fraction of observable transitions. Nevertheless, the observed results provide information on the rovibrational energy of the 3ν2(l=1) state which may be used to further improve the variational calculations by Miller and Tennyson.

Spectral lines and distribution of H^{+} _{3} in high rotational levels
View Description Hide DescriptionInfrared spectrum of the ν2 fundamental band of H^{+} 3 in high rotational levels has been studied. Three motives for this study were (i) to provide laboratory data for the observation of astronomical objects in which a large abundance of H^{+} 3 exists at high temperature, (ii) to study kinetic energy distribution of H^{+} 3 in plasmas and to determine its rotational and translational temperatures, and (iii) to provide information on high rovibrational states for the variational calculations on the intramolecular dynamic of H^{+} 3. In order to increase the kinetic temperature, water‐cooled plasmas with helium dominated gas mixtures with He/H2∼5/0.6 torr were used. The observed rotational level in the ground state with the highest rotational quantum numbers was J=K=15 which has the rotational energy of 5091.6 cm^{−1}. It was found from the observed relative intensities of the rovibrational transitions and linewidths that H^{+} 3 in the plasmas were in approximate thermal equilibrium with both rotational and translational temperature of ∼1000 K. On the contrary, an analysis of the relative intensities of the H^{+} 3 spectral lines observed in our previous study of carbocation spectroscopy showed nonthermal rotational distribution. A semiquantitative discussion is given on the observed results.

Collisional effects in Q branch coherent anti‐Stokes Raman spectra of N_{2} and O_{2} at high pressure and high temperature
View Description Hide DescriptionA temperature and pressure dependent study of coherent anti‐Stokes Raman scattering (CARS) Q branch spectra of molecular nitrogen and oxygen has been conducted. Spectra at pressures up to 250 MPa and in the temperature range 298 K<T<850 K have been obtained using a scanning CARS apparatus. The full‐width at half‐maximum (FWHM) as well as peak position of collapsed Q branch profiles were measured. Measurements also have been made in synthetic air and in mixtures with argon. A detailed comparison of Q branch CARS band shapes with theoretical models of quantum mechanical and quasiclassical origin has been performed. On the one hand existing scaling laws like the modified energy gap (MEG), energy corrected sudden (exponential) polynomial energy gap [ECS‐(E)P], polynomial energy gap (PEG), and statistical polynomial energy gap (SPEG) laws that give analytical expressions for rotational relaxation rates are used in a CARS code to calculate half‐widths of the collapsed Q branch of nitrogen and oxygen. Many of these models show significant deviations from experimental results in the high pressure regime investigated here. For nitrogen the PEG‐law, although not very suitable at lower densities, at room temperature reasonably reproduces the half‐widths in the high pressure regime. The same is true for the ECS‐EP law at low and high temperatures, whereas the SPEG‐law only gives reasonable results at high temperature. For oxygen only the MEG and ECS‐EP laws (at room temperature) give half‐widths that are within the error limits of the measurement. On the other hand, within experimental error frequency shifts and half‐widths of N2 and O2 CARS‐spectra are well described by the classical approach throughout the density range. It is found that dephasing contributions to the density induced spectral shift cannot be neglected at room temperature but are less important at higher temperatures. In comparison to experimental data the quasiclassical model provides physical interpretation of temperature dependent cross sections for rotational energy relaxation processes in nitrogen and oxygen at high densities.

Perylene–rare‐gas heteroclusters. I. Electronic spectroscopy
View Description Hide DescriptionIn this paper we report on the electronic two‐photon two‐color near threshold spectroscopy of mass‐resolved perylene⋅Ar_{ n } (n=1–45), perylene⋅Kr_{ n } (n=1–35), perylene⋅(N_{2})_{ n } (n=1–12), and perylene⋅(CH_{4})_{ n } (n=1–10) heteroclusters. The S _{0}→S _{1} inhomogeneously broadened spectra of perylene⋅Ar_{ n } (n=1–6) and perylene⋅Kr_{ n } (n=1–4) exhibit resolved spectral features, which were assigned on the basis of experimental combination rules and polarizability relations to the electronic origins of distinct two‐sided and one‐sided structural isomers. Larger perylene⋅A_{ n } (A=Ar, Kr; n=6–10) heteroclusters exhibit an ‘‘abnormal’’ specific size dependence of the red spectral shifts, which decrease with increasing n and reach a local minimum at n=8. Similar characteristics of the red spectral shifts are exhibited for perylene⋅(N_{2})_{ n } and perylene⋅(CH_{4})_{ n } (n=4–8) heteroclusters. This abnormal size dependence of the spectral shifts is attributed to the dominance of one‐sided single‐layered and double‐layered structural isomers in this cluster size domain. On the basis of the comparison between the spectroscopic data and molecular dynamics simulations of the absorptionline shapes we have obtained a quantitative description of isomer‐specific structures for n=2–6, a semiquantitative description of the abnormal size domain for n=6–10 (due to the dominance of one‐sided structures with the abundance of two‐layered structures increasing at higher n), the prevalence of one‐sided structures for n=16 and n=22, and the realization of two‐sided multilayered structures at n=45.