br Acknowledgment The study was supported by
Acknowledgment The study was supported by the grant of the President of the Russian Federation (the section corresponding to Fig. 2), the Russian Science Foundation Grant 14-25-00024 (the section corresponding to Fig. 3), and the state grant 17.1360.2014/K (the section corresponding to Fig. 4)
The greenhouse effect has been the subject of much discussion recently, even though it was discovered quite a long time ago (see, for example, Ref. ). In brief, the phenomenon can be summarized thus. Proceeding from Planck\'s formula for thermal radiation and considering the temperature of the Sun to be equal to 6000K, the solar radiation that passes through the Earth\'s atmosphere and heats it up lies within the visible and the near-infrared (IR) regions. The wave numbers of this band are in the range of about 3000–25,000cm (the atmosphere is the transparent glass of the greenhouse). Radiation from the Earth\'s heated surface at room temperature (290K) lying within the mid- and longwave IR regions (approximately 200–2000cm) is not transmitted through the atmosphere, and the heat is trapped. This leads to a temperature increase in the ‘greenhouse’ compared with the open ground. Let us examine this phenomenon more closely. It is known that nitrogen (N) and oxygen (O) are the main components of the Earth\'s atmosphere (making up 78% and 21% of its volume, respectively) . According to the classification of point symmetry groups, these diatomic molecules belong to the group, i.e., they have a center of symmetry. The symmetry of the equilibrium configuration of the molecule is preserved under the reflection operation (called inversion), and therefore these molecules do not have a dipole moment. The latter does not emerge when the molecules vibrate and rotate, so, consequently, they are characterized by an absence of infrared pdk1 and emission. Thus, the main absorbents in the Earth\'s atmosphere are (in order of importance) water vapor and carbon dioxide. According to Ref. , the change in relative humidity in the Earth\'s atmosphere lies in the range of 6%–85%. For example, the average annual relative humidity in St. Petersburg reaches 80% , with 67% in the summer. As for carbon dioxide, its percentage (by volume) is only 0.03% . During the year, it varies only slightly. For example, for the whole 2013, this change was only 0.0004% (the data of the National Oceanic and Atmospheric Administration, USA). However, even though carbon dioxide\'s content in the atmosphere is so low, the considerable optical thickness of the atmosphere should be taken into account (the troposphere layer is about 10km thick), and, therefore, carbon dioxide cannot be excluded from consideration. Water and carbon dioxide molecules belong to the and point symmetry groups, respectively. corresponds these molecules to active mode frequencies (in wave numbers) and their symmetry types in IR absorption and emission. In contrast to the water molecule, the carbon dioxide molecule has a center of symmetry. The dipole moment does not change under symmetric vibrations () relative to the center of symmetry, and therefore the IR absorption and emission are absent. The dipole moment occurs and generates these spectra under asymmetric () and doubly degenerate deformation () vibrations with the respective and symmetries. Recall that the letters and indicate, respectively, a non-degenerate and a doubly degenerate vibrations, and the letter stands for the sign reversal of the displacement of atoms from their equilibrium positions (for the vibration under consideration) under inversion. For the water molecule the letters and , as well as the numbers 1 and 2, indicate, respectively, the symmetry (sign preserved: and 1) and the asymmetry (sign reversed: and 2) of the displacement of atoms from their equilibrium position (for the vibration under consideration) relative to the second-order symmetry axis (i.e., for a 180° turn), and with a reflection in the plane perpendicular to the molecular plane and passing through this symmetry axis. The IR absorption spectrum of the water molecule is well-studied . Along with the fundamental frequencies (tones) , and (see ), a series of overtones and combined frequencies is observed not only in the infrared, but also in the visible range (). For example, it can be seen from that the transition from the first overtone band of the symmetric deformation vibration (0, 2, 0) to the combined frequency band (2, 0, 3) corresponds to the change in frequency from 3152 to 17,495cm. These bands correspond to hundreds of vibrational-rotational lines. For example, a single absorption band of 3500–4000cm is formed by overlapping stretching bands of 3657 and 3756cm. Due to the presence of these vibrations, water vapor absorbs a significant percentage of the solar IR radiation (inter alia) . Thus, the region in question, which should seemingly be transparent to solar radiation, which is to say, serve as a ‘greenhouse glass’, in fact greatly overlaps with the opaque region. Therefore, the Earth\'s atmosphere does not act as a ‘glass’, and the terms ‘greenhouse effect’ and ‘greenhouse gases’ lose their original meanings; they are only used to refer to the problem by tradition.