def emission_spectrum(spectrum_data):
    """Takes spectral data of energy eigenvalues and returns the emission spectrum relative to a state
    of given index. The resulting "upwards" transition frequencies are calculated by subtracting from eigenenergies
    the energy of the select state, and multiplying the result by -1. Resulting negative
    frequencies, corresponding to absorption instead, are omitted.

    Parameters
    ----------
    spectrum_data: SpectrumData

    Returns
    -------
    SpectrumData object
    """
    spectrum_data.energy_table *= -1.0
    spectrum_data.energy_table = spectrum_data.energy_table.clip(min=0.0)
    return spectrum_data