Terahertz to kilohertz: one line, two literatures
What this conversion does
It multiplies by a billion, taking a line position out of the optical notation and into the radio one. The same molecular transition can appear in a microwave spectroscopy paper as a frequency in kilohertz and in a far-infrared paper as a position in terahertz. Converting lets a measurement from one field be checked against the other without re-deriving anything.
The fingerprint is the pattern
A molecule absorbs at a set of frequencies determined by its rotational energy levels, and the spacing between adjacent lines depends on the molecule's moment of inertia. That spacing is the fingerprint: it identifies the molecule and, with enough resolution, distinguishes one isotopic form from another. Reading the pattern requires the lines to be comparable, which is why a table of them is usually converted into a single unit before anyone tries to interpret it.
Terahertz to kilohertz conversion table
Laboratory frequencies in both units, laid out so the regularity of a rotational progression can be seen in the numbers themselves.
| Terahertz (THz) | Kilohertz (kHz) |
|---|---|
| 1 | 1000000000 |
| 2 | 2000000000 |
| 10 | 10000000000 |
| 100 | 100000000000 |
| 300 | 300000000000 |
| 1000 | 1000000000000 |
Why both notations survive
Spectrometers are built by two communities with different instruments. The one tunes a source and records power against frequency, so its output is naturally in kilohertz or megahertz. The other disperses radiation and records intensity against position, so its output is naturally in terahertz. Neither is going to change, which means the conversion is what makes the two bodies of published data usable together.
Nine places, and the fraction
Multiplying by a billion moves the decimal point nine places to the right, so 0.5 becomes 500,000,000. A value with three decimals in terahertz comes out with three decimals in kilohertz as well, because a power of ten shifts the point without changing the digits.
Frequently asked questions
How do I convert terahertz to kilohertz?
Multiply by a billion. So 0.5 THz is 500,000,000 kHz. The conversion is three prefix steps — megahertz, gigahertz, terahertz — so the decimal point moves nine places.
Why would a spectral line be quoted in two different units?
Because the two instrument traditions record different quantities. A tuned source produces a frequency reading, naturally in kilohertz or megahertz; a dispersive instrument produces a position reading, naturally in terahertz. Published data therefore comes in both.
What does a rotational spectrum look like?
A series of lines at fairly regular spacing. The spacing is set by the molecule's rotational inertia, so measuring it identifies the molecule and can distinguish between isotopic forms that differ only in the mass of one atom.
Can the converter handle a small terahertz value?
Yes. A line at 0.05 THz converts to 50,000,000 kHz, and the result is shown in full rather than abbreviated. Inputs are accepted with several decimal places without any loss of precision.
Is a billion kilohertz the same as a terahertz?
Yes. One terahertz is a thousand gigahertz, one gigahertz is a thousand megahertz, and one megahertz is a thousand kilohertz, so a billion kilohertz is exactly one terahertz.