Ground-state Species Research Articles

Isomerization dynamics of a novel cis/trans‐only merocyanine

AbstractMerocyanines (MC) usually adopt ring opened zwitterionic structures that are interconvertible with their ring‐closed spiropyran photoisomers. By methylating the phenolate oxygen, and thereby blocking the ring‐closure reaction, a cis/trans‐only MC photoswitch was obtained, yielding a perfect candidate for a detailed examination of the cis/trans isomerization mechanism for this class of compounds. This photoswitch displays outstanding properties including excellent photoreaction quantum yields and photoswitching turnovers. Due to the central polymethine bridge of MC, in principle eight cis (C)/trans/(T) isomers are possible. Density Functional Theory (DFT) calculations revealed the CCT and TTT‐isomers of the studied compound as most stable cis and trans ground state isomers, respectively. UV/vis transient absorption studies combined with conical intersection computations with the complete active space self‐consistent field (CASSCF) method show that both trans/cis‐ and cis/trans‐photoisomerizations are initiated by a rotation of the central doubled bond fragment. A hot ground state species is then formed, which undergoes a second isomerization. Thus, the cis/trans reaction proceeds via a CCT‐CTT‐TTT sequence and the reverse reaction via TTT‐TCT‐CCT.

Unraveling the effect of aromaticity for the dynamics of excited states of single benzene fluorophores.

The photophysical properties of a series of recently synthesized single benzene fluorophores were investigated using ensemble density functional theory calculations. The energetic stability of the ground and excited state species were counterposed against the aromaticity index derived from local vibrational modes. It was found that the large Stokes shift of the fluorophores (up to ca. 5800 cm ) originates from the effect of electron donating and electron withdrawing substituents rather than -delocalization and related (anti-)aromaticity. On the basis of nonadiabatic molecular dynamics simulations, the absence of fluorescence from one of the regioisomers was explained by the occurrence of easily accessible S /S conical intersections below the vertical excitation energy level. It is demonstrated in the manuscript that the analysis of local mode force constants and the related aromaticity index represent a useful tool for the characterization of -delocalization effects in -conjugated compounds.

Photochemical formation of the elusive Dewar isomers of aromatic systems: why are substituted azaborines different?

Photochemical reactions enabling efficient transformation of aromatic systems into energetic but stable non-aromatic isomers have a long history in organic chemistry. One recently discovered reaction in this realm is that where derivatives of 1,2-azaborine, a compound isoelectronic with benzene in which two adjacent C atoms are replaced by B and N atoms, form the non-hexagon Dewar isomer. Here, we report quantum-chemical calculations that explain both why 1,2-azaborine is intrinsically more reactive toward Dewar formation than benzene, and how suitable substitutions at the B and N atoms are able to increase the corresponding quantum yield. We find that Dewar formation from 1,2-azaborine is favored by a pronounced driving force that benzene lacks, and that a large improvement in quantum yield arises when the reaction of substituted 1,2-azaborines proceeds without involvement of an intermediary ground-state species. Overall, we report new insights into making photochemical use of the Dewar isomers of aromatic compounds.

The carbon-iodine bond cleavage and isomerization of iodoform visualized with femtosecond X-ray liquidography.

Iodoform (CHI3) has garnered significant attention for its unique ability to induce photo-cyclopropanation of olefins by releasing an iodine radical through C-I bond cleavage. However, the detailed mechanism underlying CHI3 photodissociation is still not fully understood. Here, we elucidate the ultrafast structural dynamics of CHI3 upon photoexcitation using femtosecond time-resolved X-ray liquidography (fs-TRXL) at an X-ray free-electron laser facility. The fs-TRXL data was decomposed into the isotropic and anisotropic data. The isotropic data reveal that the formation of CHI2 and I radicals upon photolysis precedes the emergence of iso-CHI2-I. After a short induction period, two competing geminate recombination pathways of CHI2 and I radicals take place: one pathway leads to the recovery of CHI3, while the other results in the formation of iso-CHI2-I. Additionally, the anisotropic data show how the transient anisotropic distribution of both the species formed upon photoexcitation and the ground-state species depleted upon photoexcitation decays through rotational dephasing. Furthermore, the observed structural dynamics of CHI3 has distinctive differences with that of BiI3, which can be attributed to differences in their central moieties, CH and Bi. Our findings provide insights into the photoinduced reaction dynamics of CHI3, enhancing the understanding of its role in photochemical reactions.

Spectroscopy and Photochemistry of OAlNO and Implications for New Metal Chemistry in the Atmosphere.

A new aluminum-bearing species, OAlNO, which has the potential to impact the chemistry of the Earth's upper atmosphere, is characterized via high-level, ab initio, spectroscopic methods. Meteor-ablated aluminum atoms are quickly oxidized to aluminum oxide (AlO) in the mesosphere and lower thermosphere (MLT), where a steady-state layer of AlO then builds up. Concurrent formation of nitric oxide (NO) in the same region of the atmosphere will lead to the bimolecular formation of the OAlNO molecule. Molecular orbital analysis provides fundamental insights into the chemical bonding and energetic arrangement of the triplet (1 3A″) ground state and singlet (1 1A') excited-state species of OAlNO. Additionally, unpaired electrons on the terminal oxygen atom of triplet (1 3A″) OAlNO cause it to be reactive to atmospheric species, potentially impacting climate science and high-altitude chemistry. The triplet (1 3A″) ground-state species exhibits a large permanent dipole moment useful for rotational spectroscopic detection; however, similar rotational constants to the singlet (1 1A') excited-state species will hamper differentiation in a spectrum. Strong infrared intensities will assist in detection and discrimination of the different spin states and isomers. Repulsive electronic excited states of OAlNO will lead to photolysis of the Al-N bond and formation of various electronic states of AlO + NO through nonadiabatic pathways. Reaction through the OAlNO intermediate represents a means for the production of electronically excited AlO, leading to new chemistry in the atmosphere. Excitation to higher-lying electronic states will lead to fluorescence with a minor Stokes shift, useful for laboratory investigation. Such physical properties of this molecule will allow for new, unexplored chemical pathways in the MLT to be considered.

Excited-State-Selective Ultrafast Relaxation Dynamics and Photoisomerization of trans-4,4'-Azopyridine.

Excited-state dynamics of trans-4,4'-azopyridine in ethanol is studied using femtosecond transient absorption with 30 fs temporal resolution. Exciting the system at three different wavelengths, 460 and 290 (275) nm, to access the S1 nπ* and S2 ππ* electronic states, respectively, reveals a 195 cm-1 vibrational coherence, which suggests that the same mode is active in both nπ* and ππ* relaxation channels. Following S1-excitation, relaxation proceeds via a nonrotational pathway, where a fraction of the nπ* population is trapped in a planar minimum (lifetime, 2.1 ps), while the remaining population travels further to a second shallow minimum (lifetime, 300 fs) prior to decay into the ground state. Population of the S2 state leads to 30 fs nonrotational relaxation with a concurrent buildup of nπ* population and nearly simultaneous formation of hot ground-state species. An increase in the cis-isomer quantum yield upon ππ* versus nπ* excitation is observed, which is opposite to trans-azobenzene.

Unique CT emission from aryl terpyridine nanoparticles in aqueous Medium: A combined effect of excited state hydrogen bonding and conformational planarization

Two conformationally flexible aryl terpyridine molecules (aryl: pyrene and anthracene) have been synthesized that can form nanoscopic aggregates in aqueous medium. Unlike organic medium, these bichromophoric molecules showed broad, red-shifted multiple emission bands when microinjected in water. Experimental evidences indicate this unique emission behavior from nanoparticles originating from only a single ground state species. The existence of multiple emission bands and the large bathochromic shifts suggesting planarization of excited state induced by aggregation of chromophores. At the same time, due to higher basicity of terpyridine nitrogens at excited state (photo-basicity), the molecules could be involved H-bonding with water molecules. This could further stabilize a unique internally decoupled charge transfer (CT) state, as evident by time-dependent fluorescence studies. Intermolecular CT promoted by aggregation and mediated by H-bonding of terpyridine to water is indeed a rare example. Though the aggregate formation was found to be insensitive over a large pH range, a unique three-way optical switch can be constructed using metal ions (Zn2+ ions) as additive.

Laser absorption spectroscopy on a transient aluminum plasma generated by excimer laser ablation

Transient Al plasma produced by excimer laser ablation in vacuum was investigated through laser absorption spectroscopy to probe the space-time evolution of neutral species. Spectrally integrating the absorbance, the ground state Al atoms column density was calculated. Typical decreasing trends in time (over a plasma lifetime of about 10 μs) and space (for distances in the range 0.5–8.5 mm from the target surface) were observed. The kinetic temperature of Al ground state species was also estimated from the recorded linewidths, in the assumption of a Doppler broadening mechanism. The resulted values displayed only weak time-fluctuations, while the time-averaged ones showed. a significant drop in few millimeters from the target, similarly to the column density behavior.

Ultrafast infrared transient absorption spectroscopy of gas-phase Ni(CO)4 photodissociation at 261 nm.

We employ ultrafast mid-infrared transient absorption spectroscopy to probe the rapid loss of carbonyl ligands from gas-phase nickel tetracarbonyl following ultraviolet photoexcitation at 261nm. Here, nickel tetracarbonyl undergoes prompt dissociation to produce nickel tricarbonyl in a singlet excited state; this electronically excited tricarbonyl loses another CO group over tens of picoseconds. Our results also suggest the presence of a parallel, concerted dissociation mechanism to produce nickel dicarbonyl in a triplet excited state, which likely dissociates to nickel monocarbonyl. Mechanisms for the formation of these photoproducts in multiple electronic excited states are theoretically predicted with one-dimensional cuts through the potential energy surfaces and computation of spin-orbit coupling constants using equation of motion coupled cluster methods (EOM-CC) and coupled cluster theory with single and double excitations (CCSD). Bond dissociation energies are calculated with CCSD, and anharmonic frequencies of ground and excited state species are computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT).

Collisional Radiative Modeling of Electronically Excited States in a Hypersonic Flow

The hypersonic, nonequilibrium thermochemistry flow over a cylinder was modeled to study nitric oxide (NO) ultraviolet (UV) emissions. The main focus of this work is to model the populations of the electronically excited states of and NO because the transitions from those states to ground states are responsible for UV emissions. Collisional radiative models were constructed for the purpose of predicting the electronically excited state populations using two approaches, the quasi-steady-state (QSS) method and a MassTR method. These methods were applied to flows generated by the direct simulation Monte Carlo method of ground state species for two types of collisional radiative models. The results showed that when the coupling between the formation of electronically excited states and flow transport was modeled, this resulted in higher populations of the electronically excited states of () and NO in the expansion and wake regions of the flow compared to the QSS result where flow transport is not included. In addition, it was shown that the () state is responsible for the most of the NO(A) state population downstream of the stagnation region.

Do not forget the Rydberg orbitals.

Within any molecule or cluster containing one or more positively charged sites, families of Rydberg orbitals exist. Free electrons can attach directly, and anionic reagents with low electron binding energy can transfer an electron into one of these orbitals to form a neutral Rydberg radical. The possibilities that such a radical could form a covalent bond either to another Rydberg radical or to a radical holding its electron in a conventional valence orbital are considered. This Perspective overviews two roles that Rydberg radicals can play, both of which have important chemical consequences. Attachment of an electron into excited Rydberg orbitals is followed by rapid (∼10-6s) relaxation into the lowest-energy Rydberg orbital to form the ground state radical. Although the excited Rydberg species are stable with respect to fragmentation, the ground-state species is usually quite fragile and undergoes homolytic bond cleavage (e.g., -R2NH dissociates into -R2N + H or into -RNH + R) by overcoming a very small barrier on its potential energy surface, thus generating reactive radicals (H or R). Here, it is shown that as a result of this fragility, any covalent bonds formed by Rydberg radicals are weak and the molecules they form are susceptible to exothermic fragmentations that involve quite small activation barriers. Another role played by Rydberg species arises when the Coulomb potentials provided by the (one or more) positive site(s) in the molecule stabilize low-energy anti-bonding orbitals (e.g., σ* orbitals of weak σ bonds or low-lying π* orbitals) to the extent that electron attachment into these Coulomb-stabilized orbitals is rendered exothermic. In such cases, the overlap of the Rydberg orbitals on the positive site(s) with the σ* or π* orbitals allows either a free electron or a weakly bound electron to an anionic reagent that is attracted toward the positive site by its Coulomb force to be guided/transferred into the σ* or π* orbital instead. After attaching to such an anti-bonding orbital, bond cleavage occurs again, generating reactive radical species. Because of the large radial extent of Rydberg orbitals, this class of bond cleavage events can occur quite distant from the positively charged group. In this Perspective, several examples of both types of phenomena are given for illustrative purposes.

Excimer Formation of Perylene Bisimide Dyes within Stacking-Restrained Folda-Dimers: Insight into Anomalous Temperature Responsive Dual Fluorescence

Excimer Formation of Perylene Bisimide Dyes within Stacking-Restrained Folda-Dimers: Insight into Anomalous Temperature Responsive Dual Fluorescence

Iron‐Iron Bond Lengths and Bond Orders in Diiron Lantern‐Type Complexes with High Spin Ground States

AbstractDiiron paddlewheel‐ or lantern‐type complexes comprise an interesting subclass of binuclear iron complexes, existing with digonal, trigonal, and tetragonal ligand arrays. Experimentally known members show Fe−Fe bonds of lengths ranging from 2.13 to 2.73 Å, with Fe−Fe bond orders ranging from 0.5 to 2. Truncated models for 30 experimentally characterized diiron paddlewheel‐type complexes have been studied by DFT using the M06‐L functional, reproducing the Fe−Fe bond lengths quite well. In addition, we use DFT to treat three series of model diiron complexes Fe2Lx (x=2, 3, 4) in various low‐lying spin states, L being the unsubstituted formamidinate, guanidinate, and formate ligands (along with a series of axially ligated formate complexes) in order to predict ground state spin multiplicities, Fe−Fe bond lengths, and features of the ligand arrays. The ground states all have high spin multiplicities (septets, octets, and nonets). Formal bond order (fBO) values are suggested for the Fe−Fe bonds in these 61 complexes using an electron bookkeeping procedure, in addition to the Fe−Fe bond orders obtained by metal‐metal MO analysis for ground state species. Fe−Fe bond orders up to 3 are noted in some excited states. Deviations from D3h and D4h symmetry are noted for many trigonal and tetragonal complexes, being attributed to inherent Jahn‐Teller distortions. The formamidinate and guanidinate series show many similarities, while the formate series differs from these two in several aspects. From these results, ranges are derived for Fe−Fe bond lengths of orders 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0. The Fe−Fe bond length ranges for these non‐carbonyl lantern complexes are found to be appreciably lower than the corresponding ranges for diiron complexes with carbonyl ligands as compiled earlier from computational and experimental results.

Molecular Recognition in Water Using Macrocyclic Synthetic Receptors.

Molecular recognition in water using macrocyclic synthetic receptors constitutes a vibrant and timely research area of supramolecular chemistry. Pioneering examples on the topic date back to the 1980s. The investigated model systems and the results derived from them are key for furthering our understanding of the remarkable properties exhibited by proteins: high binding affinity, superior binding selectivity, and extreme catalytic performance. Dissecting the different effects contributing to the proteins' properties is severely limited owing to its complex nature. Molecular recognition in water is also involved in other appreciated areas such as self-assembly, drug discovery, and supramolecular catalysis. The development of all these research areas entails a deep understanding of the molecular recognition events occurring in aqueous media. In this review, we cover the past three decades of molecular recognition studies of neutral and charged, polar and nonpolar organic substrates and ions using selected artificial receptors soluble in water. We briefly discuss the intermolecular forces involved in the reversible binding of the substrates, as well as the hydrophobic and Hofmeister effects operating in aqueous solution. We examine, from an interdisciplinary perspective, the design and development of effective water-soluble synthetic receptors based on cyclic, oligo-cyclic, and concave-shaped architectures. We also include selected examples of self-assembled water-soluble synthetic receptors. The catalytic performance of some of the presented receptors is also described. The latter process also deals with molecular recognition and energetic stabilization, but instead of binding ground-state species, the targets become elusive counterparts: transition states and other high-energy intermediates.

Numerical study on simplified reaction set of ground state species in CO<sub>2</sub> discharges under Martian atmospheric conditions

The exploration of Mars has attracted increasing interest in these years. The experiments and simulations show that strong electric field triggered by the dust storms in the Martian atmosphere may cause CO<sub>2</sub> discharge. Studies on this phenomenon will not only help deepen our comprehension on the evolution of Martian surface, but also provide a possibility to realize the <i>in-situ</i> oxygen generation on Mars based on plasma chemistry. In this paper, a zero-dimensional global model is used to simplify the complicated description of CO<sub>2</sub> chemical kinetics, therefore a reduced chemistry can be obtained for detailed numerical simulation in the near future. At the beginning of simplification, the graph theoretical analysis is used to pre-treat the original model by exploring the relationship between reacting species. Through the study of connectivity and the topological network, species such as O<sub>2</sub>, e, and CO prove to be important in the information transmission of the network. While gaining a clearer understanding of the chemistry model, dependence analysis will be used to investigate numerical simulation results. In this way a directed relation diagram can be obtained, where the influence between different species is quantitively explained in terms of numerical solutions. Users could keep different types of species correspondingly according to their own needs, and in this paper, some species with low activeness such as C<sub>2</sub>O, <inline-formula><tex-math id="M5">\begin{document}$ {\mathrm{O}}_{5}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M5.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M6">\begin{document}$ {\mathrm{O}}_{4}^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M6.png"/></alternatives></inline-formula> and species with uncertainties such as <inline-formula><tex-math id="M7">\begin{document}$ {\mathrm{C}}_{2}{\mathrm{O}}_{2}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M7.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M8">\begin{document}$ {\mathrm{C}\mathrm{O}}_{4}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M8.png"/></alternatives></inline-formula> are removed from the original model. As for the reduction of specific reactions among species, the reaction proportion analysis based on the calculation of reaction rates is used to obtain the contribution of each reaction to the entire process of CO<sub>2</sub> discharge, through which the important reactions can be selected. Finally, a simplified chemistry model of CO<sub>2</sub> discharge based on Martian atmospheric conditions, including 16 species and 67 reactions, is established. The numerical simulations show that the trends of species densities based on the simplified chemistry model are highly consistent with those based on the original one, and considerations about the CO<sub>2</sub> conversion and the energy efficiency are also in line with expectations, which can help deepen the understanding of the essential process of CO<sub>2</sub> discharge under Martian atmospheric conditions. In addition, the quantitative results of the relationship between reactive species will lay a theoretical foundation for the accurate analysis of various products in Martian dust storm discharges and the realization of Mars <i>in-situ</i> oxygen generation technology based on plasma chemistry.

Binuclear Cobalt Paddlewheel-Type Complexes: Relating Metal-Metal Bond Lengths to Formal Bond Orders.

Paddlewheel-type complexes are prominent among experimentally known binuclear cobalt complexes and incorporate substituted formamidinate, guanidinate, and carboxylate ligands in digonal, trigonal, and tetragonal arrays around the bimetallic core. Such complexes are modeled here by density functional theory using unsubstituted ligands, extending the whole set to incorporate a variety of metal oxidation states and spin multiplicities. The DFT results for ground state cobalt-cobalt bond lengths and ground state spin multiplicity of the model complexes are often quite close to the experimental results for the corresponding substituted complexes. The three series of complexes often exhibit parallel trends with regard to effects of change in the metal oxidation state and spin multiplicity. The formamidinate and guanidinate series show marked resemblances. The lowered symmetry in many model trigonal complexes implies that such deviations in the experimentally known congeners arise from the inherent electronic structure. For ground state species, the DFT results provide Co-Co bond orders (BOs) from MO occupancy considerations. Further, using a revised electron bookkeeping method, Co-Co formal bond order (fBO) values from 0.0 to 2.0 are assigned to all of the 85 complexes studied. The computed Co-Co bond lengths fall into distinct ranges according to the formal bond order values (from 0.5 to 2).

Astromers: Nuclear Isomers in Astrophysics

We develop a method to compute thermally-mediated transition rates between the ground state and long-lived isomers in nuclei. We also establish criteria delimiting a thermalization temperature above which a nucleus may be considered a single species and below which it must be treated as two separate species: a ground state species, and an astrophysical isomer ("astromer") species. Below the thermalization temperature, the destruction rates dominate the internal transition rates between the ground state and the isomer. If the destruction rates also differ greatly from one another, the nuclear levels fall out of or fail to reach thermal equilibrium. Without thermal equilibrium, there may not be a safe assumption about the distribution of occupation probability among the nuclear levels when computing nuclear reaction rates. In these conditions, the isomer has astrophysical consequences and should be treated a separate astromer species which evolves separately from the ground state in a nucleosynthesis network. We apply our transition rate methods and perform sensitivity studies on a few well-known astromers. We also study transitions in several other isomers of likely astrophysical interest.

Copper 1,19-Diaza-21,24-dicarbacorrole: A Corrole Analogue with an N-N Linkage Stabilizes a Ground-State Singlet Organocopper Species.

A copper complex of a heterocorrole analogue with an N-N linkage, 1,19-diaza-21,24-dicarbadibenzocorrole (Cu-5), was successfully synthesized via oxidative metalation-cyclization of a tetrapyrrolic precursor. The N-N linkage in the skeleton of Cu-5, which serves as a mediator of π-electron delocalization, features an 18π aromatic system. The electronic structure of Cu-5 is best described as a ground-state singlet species stabilized by the distinct NNCC coordination core. This finding shows how the ligand's design can be used to modulate the Cu orbital energy, thereby making such compounds invaluable for copper-based catalytic applications.

A spectrophotometric study of impact of solvent, substituent and cross-conjugation in some 4-aminoantipyrine based Schiff bases.

The influences of substituent and solvent polarity on the electronic transition of seven different 4-aminoantipyrine based Schiff bases have been investigated in 14 solvents of different polarity. Reichardt'sET(30) scale has been used to propose a quantitative approach towards the relative stability of the electronic ground and excited state species. The Schiff bases can sense the polarity changes of the solvents and accordingly there appear three categories of solvents: non-polar, dipolar aprotic and polar protic. The first two categories of solvents induce negative solvatochromism and the third one induces positive solvatochromism in the molecules. Cross conjugation and geometrical configuration have been proposed to play an important role in the extent of electronic transition of the Schiff bases in various solvent media. Out of various substituent effects, field effect and resonance significantly influence the electronic environment of the compounds, as evidenced from the regression models involving change in UV-Vis absorption wavelengths 1HNMR chemical shifts and IR frequencies.

Investigating recent developments and applications of optical plasma spectroscopy: A review

Optical spectroscopy is a powerful, nonintrusive diagnostic tool that can provide unparalleled insight into fundamental plasma properties. Specifically, these techniques are widely employed to qualitatively and quantitatively characterize interactions of species within a discharge. This work is comprised of two parts: (1) a brief review of recent literature on the application of optical emission spectroscopy from the past decade, ranging from the study of atomic rare gas to more complex environmentally and technologically relevant plasma systems and (2) the presentation of new data that illustrate the power of optical spectroscopy techniques beyond simple species identification. Specifically, time-resolved optical emission spectroscopy was utilized to provide kinetic information about excited state species formation, ultimately lending mechanistic insights into a range of plasma processes. In addition, by combining optical emission and broadband absorption spectroscopies, rotational and vibrational temperatures for both excited and ground state species were determined. These data provide a thermodynamic base for enhanced understanding of the fundamental chemistry in plasma systems. The two platforms explored here were plasma-assisted catalysis systems containing NxOy species and fluorocarbon plasmas utilizing a range of precursors to evoke either etching or deposition, depending on the plasma conditions.